Synthroid is a brand-name prescription medication. It’s FDA-approved for use in people with hypothyroidism and in people with a certain type of thyroid cancer. Below, we briefly describe these conditions.

Synthroid for hypothyroidism

With hypothyroidism, your body isn’t making enough thyroid hormones. These hormones are very important because they affect your metabolism. (“Your metabolism” refers to all of the chemical reactions in your body. These reactions affect everything from your digestion to your body’s growth and repair to even your temperature.) Synthroid increases your body’s thyroid hormone levels back to a normal range.

Synthroid is approved to treat the three main types of hypothyroidism:

  • primary (caused by problems in your thyroid gland)
  • secondary (caused by problems in your pituitary gland)
  • tertiary (caused by problems in your hypothalamus)

Synthroid is approved to treat hypothyroidism that’s congenital (present at birth) or acquired (developed later in life).

For more detail on hypothyroidism, see the “Synthroid uses” section below.

Synthroid for TSH suppression in people with a certain type of thyroid cancer

Synthroid is approved for use in people with thyrotropin-dependent well-differentiated thyroid cancer. For this condition, Synthroid is used to suppress (lower) your levels of a hormone called thyroid stimulating hormone (TSH). When given as part of thyroid cancer treatment, Synthroid is used along with radioactive iodine treatment and surgery.

For more details on using Synthroid for this type of thyroid cancer, see the “Synthroid uses” section below.

Drug limitations

The manufacturer of Synthroid has stated that the drug isn’t approved for the following uses:

Drug details

Synthroid contains the active drug levothyroxine. It comes as tablets that are taken by mouth, and it’s available in several strengths. For information about the available strengths of Synthroid, see the “Synthroid dosage” section below.

Synthroid is approved for use in people of all ages who have reduced thyroid function. This includes infants, children, adolescents, and adults.

Effectiveness

For information about Synthroid’s effectiveness, see the “Synthroid uses” section below.

Synthroid contains one active drug ingredient: levothyroxine.

Synthroid is available as a generic drug called levothyroxine. A generic drug is an exact copy of the active drug in a brand-name medication. The generic is considered to be as safe and effective as the original drug. Generics tend to cost less than brand-name drugs.

Switching between Synthroid and its generic versions

It’s important to note that Synthroid and generic levothyroxine may each have different effects on your thyroid hormone levels. Because of this, it’s recommended by the American Thyroid Association and other organizations that you not switch between them. You should keep taking whichever medication you initially start taking.

For example, if you start taking brand-name Synthroid, you should continue to use brand-name Synthroid. If you must switch from the brand-name drug to a generic form, or vice versa, it’s recommended that you have a blood test 6 weeks after making the switch. This testing is done to be sure that your thyroid hormone levels stay within a normal range.

Even if you switch between different generic forms of levothyroxine that have different manufacturers, it’s still recommended that you have your thyroid levels tested 6 weeks later.

In some cases, brand-name Synthroid and its generic versions may come in different forms and strengths.

Be sure to talk with your doctor before switching to or from brand-name Synthroid.

Synthroid can cause mild or serious side effects. The following lists contain some of the key side effects that may occur while taking Synthroid. These lists don’t include all possible side effects.

It’s important to note that most of the side effects of Synthroid are caused by having too much thyroid hormone in your body. Even small changes in your Synthroid dose may cause large changes in your hormone levels, which may cause side effects. Be sure to talk with your doctor about any side effects you’re having with Synthroid. In some cases, your doctor may need to adjust your dosage of the drug.

Note: The Food and Drug Administration (FDA) tracks side effects of drugs it has approved. If you would like to report to the FDA a side effect you’ve had with Synthroid, you can do so through MedWatch.

Mild side effects

Mild side effects of Synthroid can include:*

Most of these side effects may go away within a few days or a couple of weeks. But if they become more severe or don’t go away, talk with your doctor or pharmacist.

* This is a partial list of mild side effects from Synthroid. To learn about other mild side effects, talk with your doctor or pharmacist, or visit Synthroid’s prescribing information.

Serious side effects

Serious side effects from Synthroid aren’t common, but they can occur. Call your doctor right away if you have serious side effects. Call 911 if your symptoms feel life threatening or if you think you’re having a medical emergency.

Serious side effects and their symptoms can include:

Side effects in children

Children taking Synthroid may have different side effects than do adults taking the drug. For example, some children taking Synthroid have had pseudotumor cerebri (increased pressure in the brain). This condition may cause headaches, nausea, and vomiting.

In addition, some children taking Synthroid have had slipped capital femoral epiphysis. (This condition is a hip disorder that causes the ball and socket joint in the hip to slip backward.) This disorder may cause limping or pain in the area that’s affected.

This hip condition only occurs in children whose bones are still growing. However, it’s important to note that having decreased thyroid hormone levels, which Synthroid is used to treat, may also increase the risk of this hip disorder.

Also, certain bone growth problems may occur in infants and children who take too much Synthroid. In this case, an infant’s skull may become hardened too quickly, causing increased pressure on their brain. And the growth of their skull may also be affected. Other bones in children taking too much Synthroid may also be unable to grow properly. This can lead to growth problems and decreased height.

If you have questions about side effects of Synthroid in a child using the drug, talk with your child’s doctor. They’ll discuss with you the risks and benefits of treatment. And they can help prepare you for what to expect from treatment with this drug.

Side effects of too much or too little Synthroid

During Synthroid treatment, getting your thyroid hormone to the right level can sometimes be challenging. This is because even a small change in your Synthroid dose may cause your thyroid hormone level to become either too high or too low. And having either too much or too little Synthroid in your body may result in certain side effects.

For example, if you take too much Synthroid, you may have side effects of hyperthyroidism (a condition that causes high thyroid hormone levels). Symptoms of hyperthyroidism may include:

  • fast heart rate
  • feeling nervous or anxious
  • heart palpitations (feeling your heart flutter or skip a beat)
  • trouble sleeping
  • muscle weakness
  • weight loss

On the other hand, if you take too little Synthroid, your body may not have enough thyroid hormone. This can lead to symptoms of hypothyroidism (a condition that causes low thyroid hormone levels). These symptoms include:

  • tiredness
  • constipation
  • weight gain
  • muscle weakness
  • hair loss

Maintaining a consistent level of Synthroid in your body can help prevent side effects of having too much or too little of the drug in your body. To do this, it’s very important that you take your Synthroid dose regularly, once each day. And you should try to take the drug at the same time every day.

Side effect details

You may wonder how often certain side effects occur with this drug. Here’s some detail on several of the side effects this drug may cause.

Weight loss

In some cases, Synthroid may cause weight loss. This is because thyroid hormones (which Synthroid is used to replace or add to) are responsible for your body’s metabolism.* If you don’t have enough thyroid hormones in your body, your metabolism will be slower than usual. And this may lead to weight gain.

However, if you’re taking Synthroid, your thyroid hormone level should be within a normal range. So with Synthroid treatment, your metabolism rate may be increased, and you may lose weight.

it’s important to note that even though Synthroid may cause weight loss, the drug shouldn’t be used specifically to aid in weight loss. In fact, Synthroid has a boxed warning advising against using the drug for weight loss. A boxed warning is the strongest warning required by the Food and Drug Administration (FDA). Boxed warnings alert doctors and patients about drug effects that may be dangerous.

Synthroid’s boxed warning states that it shouldn’t be used to treat obesity or to help with weight loss. If your thyroid function is normal, Synthroid won’t help you lose weight.

In fact, using the drug at typical dosages in people without certain thyroid problems won’t lead to weight loss. And using doses larger than those recommended by the manufacturer may cause serious side effects. These side effects can include confusion, seizures, coma, and even death. In addition, taking Synthroid along with drugs used for weight loss may cause very serious or even life threatening side effects.

If you have concerns or questions about weight loss while you’re taking Synthroid, talk with your doctor. They can recommend ways to help you manage a body weight that’s healthy for you.

* “Your metabolism” refers to all of the chemical reactions in your body. These reactions affect everything from your digestion to your body’s growth and repair to even your temperature.

How common is weight loss with Synthroid?

There isn’t any clinical study information that shows how many people lost weight while taking Synthroid. However, a small study looked at how many people lost weight while taking levothyroxine (the active drug in Synthroid).

This study showed that 52% of people taking levothyroxine lost weight after taking the drug for up to 24 months. And the people’s average weight loss was 3.8 kilograms (about 8.4 pounds).

Weight gain

In some cases, Synthroid may lead to weight gain. This is because the drug can increase your appetite, which may make you eat more food than usual.

It’s also possible for you to gain weight if your dose of Synthroid isn’t high enough for your body’s needs. This is because if your thyroid hormone levels aren’t high enough, your metabolism* may be slowed. And this might make it easier for you to gain weight. (Keep in mind that Synthroid is used to replace or add to your natural thyroid hormones.)

But there’s no information from clinical studies that shows how many people have gained weight while taking Synthroid.

If you have concerns about weight gain while you’re taking Synthroid, talk with your doctor. They can recommend ways to help you manage a body weight that’s healthy for you.

* “Your metabolism” refers to all of the chemical reactions in your body. These reactions affect everything from your digestion to your body’s growth and repair to even your temperature.

Hair loss

Hair loss is a rare side effect of Synthroid. But there aren’t any clinical studies that show how many people have had hair loss while taking this drug.

When hair loss does occur with Synthroid, it’s generally partial (affecting just patches on your scalp, rather than your entire scalp). And the hair loss typically happens during the first few months of Synthroid treatment. Usually, hair loss from Synthroid is only temporary, and it goes away even when treatment is continued.

Keep in mind that hair loss can also occur because of changes in your thyroid hormone levels. (And Synthroid is used to replace or add to natural thyroid hormones in your body.) If your hormone levels are either too high or too low, your hair may not grow properly.

If you have hair loss while you’re taking Synthroid, talk with your doctor. They may be able to recommend ways to help decrease this side effect. Your doctor may also order certain blood tests to check your thyroid hormone levels. This helps your doctor to be sure that the levels aren’t too high or too low.

Diarrhea or constipation

Diarrhea is a possible side effect of Synthroid. But it’s not known how many people taking Synthroid have had diarrhea from the drug.

However, if you have diarrhea that’s severe or bothersome while you’re taking Synthroid, talk with your doctor. They may be able to recommend ways to help decrease this side effect.

Constipation, on the other hand, is unlikely to be a side effect of Synthroid. In fact, it wasn’t reported as a side effect in people taking the drug during studies.

However, it’s important to note that constipation may be caused by hypothyroidism, which Synthroid is used to treat. This is because with low thyroid hormone levels caused by hypothyroidism, your digestion is slowed down. And this may lead to constipation.

If you have constipation while you’re taking Synthroid, talk with your doctor. They may order certain blood tests to check your thyroid hormone levels. And they’ll recommend ways to help treat your constipation.

Allergic reaction

As with most drugs, some people can have an allergic reaction after taking Synthroid. It’s important to note that allergic reactions to levothyroxine (the active drug in Synthroid) haven’t been reported. However, it’s still possible to have an allergic reaction to the inactive ingredients in Synthroid.

Symptoms of a mild allergic reaction can include:

  • skin rash
  • itchiness
  • flushing (warmth and redness in your skin)

A more severe allergic reaction is rare but possible. Symptoms of a severe allergic reaction can include:

  • swelling under your skin, typically in your eyelids, lips, hands, or feet
  • swelling of your tongue, mouth, or throat
  • trouble breathing

Call your doctor right away if you have a severe allergic reaction to Synthroid. But call 911 if your symptoms feel life threatening or if you think you’re having a medical emergency.

Anxiety

It’s possible to feel anxious while you’re taking Synthroid. But it’s not known how many people taking the drug have had anxiety.

Keep in mind that anxiety may also occur if your thyroid hormone levels are either too high or too low. (And Synthroid is used to replace or add to the natural thyroid hormones in your body.) So certain changes in your thyroid hormone levels may cause you to feel anxious or nervous.

If you’re having anxiety while taking Synthroid, talk with your doctor. They may order certain blood tests to check your thyroid hormone levels. This allows your doctor to be sure the levels aren’t too high or too low. Your doctor may also recommend ways to help decrease your anxiety while you’re taking Synthroid.

Insomnia

While you’re taking Synthroid, it’s possible to have insomnia (trouble falling asleep or staying asleep). But it’s not known how many people taking Synthroid have had this side effect.

However, insomnia can also occur when you have too much thyroid hormones in your body. (Keep in mind that Synthroid is used to replace or add to the natural thyroid hormones in your body.)

If you’re having trouble falling asleep or staying asleep, talk with your doctor. They may order certain blood tests to check if your thyroid hormone levels are too high. And your doctor may also recommend ways to help you sleep better.

Heart palpitations

You might have heart palpitations while you’re taking Synthroid. With heart palpitations, you may feel like your heart flutters or skips a beat. It’s not known how many people taking Synthroid have had heart palpitations.

Heart palpitations may also be caused by having too much thyroid hormones in your body. (Keep in mind that Synthroid is used to replace or add to the natural thyroid hormones in your body.) If your dose of Synthroid is too high for your body’s needs, your thyroid hormone levels may become too high. And this could cause you to have heart palpitations.

It’s important that you talk with your doctor if you have heart palpitations while you’re taking Synthroid. Your doctor will likely order blood tests to check your thyroid hormone levels. This allows your doctor to be sure that the levels aren’t too high. And your doctor may also be able to recommend ways to treat your heart palpitations.

The Synthroid dosage your doctor prescribes will depend on several factors. These include:

  • the type and severity of the condition you’re using Synthroid to treat
  • your age
  • your body weight
  • other medications that you’re taking
  • other medical conditions you may have

Typically, your doctor will start you on a low dosage. Then they’ll adjust it over time to reach the amount that’s right for you. For your dosage, your doctor will calculate the smallest dose that provides the desired effect.

If you have certain health conditions, your doctor may have you start taking a dosage of Synthroid that’s lower than the typical starting dose. For example, this may be the case if you have certain heart problems. Talk with your doctor about the starting dose of Synthroid that’s best for you.

The following information describes dosages that are commonly used or recommended. However, be sure to take the dosage your doctor prescribes for you. Your doctor will determine the best dosage to fit your needs.

Drug forms and strengths

Synthroid contains the active drug levothyroxine. It comes as tablets that are taken by mouth. And it’s available in the following strengths (given in micrograms):

25 mcg100 mcg150 mcg
50 mcg112 mcg175 mcg
75 mcg125 mcg200 mcg
88 mcg137 mcg300 mcg

Dosage for hypothyroidism

Typical dosages of Synthroid for hypothyroidism are described below.

Usually, high dosages of Synthroid aren’t needed to treat hypothyroidism. (High dosages are greater than 200 mcg of Synthroid daily.) And there’s no average dose of Synthroid. This is because Synthroid dosages are calculated based on your body’s thyroid hormone levels and your body weight.

Dosage for newly diagnosed hypothyroidism

A typical starting dose of Synthroid for newly diagnosed hypothyroidism is 1.6 micrograms of drug per kilogram of body weight (mcg/kg). This dose is taken once each day. (With newly diagnosed hypothyroidism, the condition was just diagnosed within the past few months.)

For example, if you weigh 70 kg (about 154 pounds), your starting dosage of Synthroid would be about 112 mcg each day.

Dosage adjustments for newly diagnosed hypothyroidism

It takes about 4 to 6 weeks for blood tests to show Synthroid’s full effect on your thyroid hormone levels. So after you’ve been taking the drug for about 4 to 6 weeks, your doctor will order blood tests to check these levels. If your thyroid hormone levels are still too low, your doctor may increase your daily dosage of Synthroid by 12.5 mcg to 25 mcg.

Then, after an another 4 to 6 weeks, your doctor will order blood tests again to check your thyroid hormone levels. If needed, your doctor may keep increasing your daily Synthroid dose by 12.5 mcg to 25 mcg every 4 to 6 weeks. They’ll continue doing this until your thyroid hormone levels are within a normal range.

Dosage for severe, longstanding hypothyroidism

The typical starting dosage of Synthroid for severe, longstanding hypothyroidism is 12.5 mcg to 25 mcg daily. (With severe, longstanding hypothyroidism, you’ve had the condition for several months.) The starting dosage of Synthroid for this condition is lower than it is for newly diagnosed hypothyroidism. (See the section just above for details.)

Dosage adjustments for severe, longstanding hypothyroidism

When treating severe, longstanding hypothyroidism, your doctor may order blood tests to check your thyroid hormone levels every 2 to 4 weeks. If your levels are low, your doctor may increase your daily dose of Synthroid every 2 to 4 weeks by 12.5 mcg to 25 mcg. They’ll continue to do this until your levels are within a normal range.

Dosage for TSH suppression

The usual dose of Synthroid for thyroid stimulating hormone (TSH) suppression* is 2 micrograms of drug per kilogram of body weight (mcg/kg). This dose is taken once each day.

This means that if you weigh 75 kg (about 165 pounds), your dosage of Synthroid would be 150 mcg each day.

Your Synthroid dosage for TSH suppression will depend on the severity of your condition and other health conditions you may have. Your doctor will recommend a dose of Synthroid for TSH suppression that’s best for you.

* For TSH suppression, Synthroid is used to help decrease levels of TSH in your body. TSH is made when your body needs thyroid hormones. A lower TSH level means your body doesn’t need thyroid hormones as much as a higher TSH level indicates.

Pediatric dosage

Synthroid dosing for hypothyroidism in children depends on both their body weight and their age. Children’s dosages of Synthroid are listed as micrograms of drug per kilogram of body weight (mcg/kg). Typical dosages for children are described below:

  • If your child is less than 3 months of age, they’ll take between 10 mcg/kg and 15 mcg/kg of Synthroid each day. For example, if your child is 2 months old and weighs 5 kg (about 11 pounds), they’ll take between 50 mcg and 75 mcg of Synthroid daily.
  • If your child is 3 to 6 months old, they’ll take between 8 mcg/kg and 10 mcg/kg of Synthroid each day.
  • If you child is 6 to 12 months old, they’ll take between 6 mcg/kg and 8 mcg/kg of Synthroid each day.
  • If your child is 1 to 5 years old, they’ll take between 5 mcg/kg and 6 mcg/kg of Synthroid each day.
  • If your child is 6 to 12 years old, they’ll take between 4 mcg/kg and 5 mcg/kg of Synthroid each day.
  • If your child is older than 12 years of age, but they’re still growing, they’ll take between 2 mcg/kg and 3 mcg/kg of Synthroid each day. However, if your child has stopped growing and has already gone through puberty, they’ll take the typical adult dosage of Synthroid. For more information about typical dosages of Synthroid for adults, see the sections just above.

Dosage adjustments in children

After 4 to 6 weeks of Synthroid treatment, your child’s doctor may order certain blood tests. These tests allow the doctor to check your child’s thyroid hormone levels and see if dosage changes are needed.

For example, if your child’s hormone levels are too low, their doctor may increase their dosage of Synthroid. Your child’s doctor may continue to adjust your child’s Synthroid dosage every 4 to 6 weeks as needed.

What if I miss a dose?

If you forget to take a dose of Synthroid, your thyroid hormone levels may decrease. And this may lead to certain side effects. (See the “Synthroid side effects” section above for more information.)

Because of this, it’s important that you remember to take your dose of the drug at the same time each day. In fact, it’s specifically recommended that you take Synthroid doses every morning.

If you forget to take a dose of Synthroid, call your doctor’s office. They or their medical staff will be able to recommend the best time for you to take your next dose of the drug.

To help make sure that you don’t miss a dose, try setting a reminder on your phone. A medication timer may be useful, too.

Will I need to use this drug long term?

Synthroid is meant to be used as a long-term treatment. If you and your doctor determine that Synthroid is safe and effective for you, you’ll likely take it long term.

Other drugs are available that can treat your condition. Some may be a better fit for you than others. If you’re interested in finding an alternative to Synthroid, talk with your doctor. They can tell you about other medications that may work well for you.

When considering alternatives to Synthroid, it’s important to note that doses of Synthroid aren’t equal to doses of other drugs that contain levothyroxine. And doses of brand-name Synthroid may not be equivalent to doses of generic forms of levothyroxine.

Because of this, if you’re switching from one drug to another, your doctor may order blood tests 4 to 6 weeks after you’ve switched medications. This allows your doctor to check your thyroid hormone levels and determine the correct dose of medication for you. Don’t switch from one thyroid drug to another without first talking with your doctor.

Alternatives for hypothyroidism or TSH suppression

Drugs other than Synthroid may be used to either treat hypothyroidism or provide thyroid stimulating hormone (TSH) suppression.* Examples of these other drugs include:

* For TSH suppression, Synthroid is used to help decrease levels of TSH in your body. TSH is made when your body needs thyroid hormones. A lower TSH level means your body doesn’t need thyroid hormones as much as a higher TSH level indicates.

Natural alternatives to Synthroid

You may wonder if there are natural treatment options for certain thyroid problems that Synthroid is used to treat. It’s important to note that there are no proven natural alternatives to Synthroid. If your doctor prescribes Synthroid for you, you shouldn’t substitute a natural alternative without first discussing it with your doctor.

But if you want to explore natural options, we describe below some approaches that may or may not help to improve your thyroid health:

  • Gluten-free diet. Sometimes thyroid function may be affected by factors you can control, such as your diet and nutrition intake. In one study, celiac disease (a condition that causes gluten sensitivity) was shown to be linked to hypothyroidism. Having a gluten-free diet may decrease gut-related symptoms of celiac disease. But it’s not known if doing so will help improve hypothyroidism that’s related to celiac disease. In fact, there currently aren’t any studies that show that a gluten-free diet can treat hypothyroidism.
  • Selenium supplements. A mineral called selenium may also affect your thyroid function. In one study, people with certain thyroid conditions had significantly lower levels of selenium in their body. It’s possible that increasing your selenium level through diet or supplements may decrease your risk of thyroid disease. However, there currently aren’t any studies that show that increasing selenium intake can decrease the risk of thyroid disease.
  • Vitamin D supplements. People with certain thyroid conditions may also have lower levels of vitamin D than usual. Some studies have shown that increasing your vitamin D level may even decrease symptoms of your thyroid condition. However, there currently aren’t any studies that show that vitamin D is an effective treatment for thyroid disorders.

If you have questions about using natural treatment options for a thyroid condition, talk with your doctor. They can recommend a treatment plan that’s right for you.

You may wonder how Synthroid compares with other medications that are prescribed for similar uses. Here we look at how Synthroid and levothyroxine are alike and different.

About

Brand-name Synthroid contains the active drug levothyroxine. (Levothyroxine is a man-made version of the natural thyroid hormone called T4.) And levothyroxine is also available as a generic drug.

A generic drug is an exact copy of the active drug in a brand-name medication. The generic is considered to be as safe and effective as the original drug.

Even though Synthroid and levothyroxine have the same active ingredient, the drugs may not be exactly identical. And switching between the two drugs may cause a large change in your thyroid hormone levels. Because of this, your doctor may need to monitor you if you switch from one drug to the other.

It’s best to stick with either the brand-name medication (Synthroid) or the generic medication (levothyroxine). But if you must switch between these versions, talk with your doctor. They’ll probably recommend that you have certain blood tests 4 to 6 weeks after making the switch. This allows your doctor to be sure that your thyroid hormone levels are within a normal range.

Uses

Synthroid and generic levothyroxine are both approved for the following uses:

For details on these two conditions, see the “Synthroid uses” section below.

* For thyroid stimulating hormone (TSH) suppression, Synthroid is used to help decrease levels of TSH in your body. TSH is made when your body needs thyroid hormones. A lower TSH level means your body doesn’t need thyroid hormones as much as a higher TSH level indicates.

Drug forms and administration

Both Synthroid and levothyroxine comes as tablets that are taken by mouth. And each of these drugs should be taken once daily in the morning.

Side effects and risks

Synthroid and levothyroxine both contain the active drug levothyroxine. Therefore, these medications can cause very similar side effects. For information about the possible side effects of Synthroid, see the “Synthroid side effects” section above.

Effectiveness

Synthroid and levothyroxine are both approved for TSH suppression and to treat hypothyroidism.

The use of Synthroid and levothyroxine in treating hypothyroidism has been directly compared in a clinical study. This study, done in 1997, looked at people taking either Synthroid or levothyroxine. It showed that there was no significant difference in thyroid hormone levels between the two groups of people.

But studies comparing Synthroid with levothyroxine have also shown mixed results.

For example, in one study of children, Synthroid lowered people’s TSH levels more than levothyroxine did. But in a different study of children, levothyroxine lowered people’s TSH level more than Synthroid did. (TSH levels indicate how much your body needs thyroid hormone. A lower TSH level means that thyroid hormone is being adequately replaced with treatment.

In both of these studies, Synthroid and levothyroxine were effective in treating people’s hypothyroidism. However, the mixed study results do support the recommendation that you shouldn’t switch back and forth between brand-name Synthroid and generic levothyroxine. For more information about switching between these drugs, see the “Synthroid generic” section above.

Costs

Synthroid is a brand-name drug, while levothyroxine is available as a generic drug. Generics usually cost less than brand-name drugs do.

According to estimates on GoodRx.com, Synthroid costs significantly more than levothyroxine. The actual price you’ll pay for either drug depends on your insurance plan, your location, and the pharmacy you use.

You may wonder how Synthroid compares with other medications that are prescribed for similar uses. Here we look at how Synthroid and Armour Thyroid are alike and different.

Ingredients

Synthroid contains the active drug levothyroxine. It’s a man-made version of the natural thyroid hormone called T4. Armour Thyroid, on the other hand, is a product that’s made from pigs’ thyroid glands. Armour Thyroid contains T4, but it also contains the other main thyroid hormone called T3.

Uses

Synthroid and Armour Thyroid are both prescribed for the following uses:

For details on these two conditions, see the “Synthroid uses” section below.

It’s important to note that Synthroid is approved for these uses by the Food and Drug Administration (FDA). However, Armour Thyroid isn’t FDA-approved for these uses.

* For thyroid stimulating hormone (TSH) suppression, Synthroid is used to help decrease levels of TSH in your body. TSH is made when your body needs thyroid hormones. A lower TSH level means your body doesn’t need thyroid hormones as much as a higher TSH level indicates.

Drug forms and administration

Both Synthroid and Armour Thyroid comes as tablets that are taken by mouth once daily.

Synthroid should be taken in the morning on an empty stomach, about 30 minutes to 1 hour before you eat breakfast. The manufacturer of Armour Thyroid hasn’t recommended a certain time of day to take its drug.

Side effects and risks

Synthroid and Armour Thyroid both contain medications used for thyroid conditions. Therefore, these medications can cause very similar side effects, but some different ones as well. Below are examples of these side effects.

It’s important to note that most of the side effects of both Synthroid and Armour Thyroid are due to having too much thyroid hormones in your body. Even small changes in your doses of these drugs may cause large changes in your hormone levels, which may result in side effects. Be sure to talk with your doctor about any side effects you’re having with these drugs. In some cases, your doctor may need to adjust your dosage of the drug.

Mild side effects

This list contains up to 10 of the most common mild side effects that can occur with both Synthroid and Armour Thyroid (when they’re taken individually).

Serious side effects

This list contains examples of serious side effects that can occur with both Synthroid and Armour (when they’re taken individually).

Effectiveness

Synthroid is approved for TSH suppression and to treat hypothyroidism. And although Armour Thyroid is also prescribed for these purposes, the drug isn’t FDA-approved for these uses.

The use of levothyroxine (the active drug ingredient in Synthroid) and Armour Thyroid in treating hypothyroidism has been directly compared in a clinical study. However, Synthroid and levothyroxine aren’t always equivalent to each other. So this study might not have shown the actual difference between Armour Thyroid and Synthroid.

The study showed that people taking Armour Thyroid had:

  • higher T3 and TSH levels than did people taking levothyroxine
  • lower T4 levels than did people taking levothyroxine

Keep in mind that T3 and T4 are the main thyroid hormones in your body. And TSH levels indicate how much your body needs thyroid hormones. So a higher TSH level means that your body needs more thyroid hormones than it has.

The study also showed that after using both drugs over a period of 16 weeks each, 48.6% of people preferred Armour Thyroid over levothyroxine. This may be because the people lost, on average, 4 pounds more while taking Armour Thyroid than while taking levothyroxine. In addition, the people reported having fewer thyroid symptoms when taking Armour Thyroid than when taking levothyroxine.

There is also an ongoing clinical trial comparing Armour Thyroid with levothyroxine. But remember that Synthroid and levothyroxine aren’t always equivalent to each other. So this study might not show the difference between Armour Thyroid and Synthroid.

Costs

Synthroid and Armour Thyroid are both brand-name drugs. There are currently no generic forms of either drug. Brand-name medications usually cost more than generics.

You can view cost estimates for these medications on GoodRx.com. But the actual price you’ll pay for either drug depends on your insurance plan, your location, and the pharmacy you use.

There aren’t any known interactions between Synthroid and alcohol. If you drink alcohol, talk with your doctor about whether it’s safe for you to drink while you’re taking Synthroid.

Synthroid can interact with several other medications. It can also interact with certain supplements as well as certain foods.

Different interactions can cause different effects. For instance, some interactions can interfere with how well a drug works. Other interactions can increase side effects or make them more severe.

Synthroid and other medications

Below are lists of medications that can interact with Synthroid. These lists don’t contain all the drugs that may interact with Synthroid.

Before taking Synthroid, talk with your doctor and pharmacist. Tell them about all prescription, over-the-counter, and other drugs you take. Also tell them about any vitamins, herbs, and supplements you use. Sharing this information can help you avoid potential interactions.

If you have questions about drug interactions that may affect you, ask your doctor or pharmacist.

Synthroid and diabetes drugs

If you have diabetes and you’re using certain diabetes drugs, taking Synthroid may increase your blood sugar level. This is because if taken with Synthroid, your diabetes drugs may not work as well as usual to manage your blood sugar level.

Examples of diabetes medications that may not work as well as usual if they’re taken with Synthroid include:

If you’re using certain diabetes drugs and you’re taking Synthroid, your doctor will recommend that you monitor your blood sugar levels more often than usual. Doing this will help to ensure that your diabetes medications are working properly, and your blood sugar level isn’t getting too high.

In some cases, your doctor may increase the dosage of your diabetes drugs. Or they may recommend that you take additional diabetes medications.

It’s possible that the dosage of any diabetes medication may need to be changed when you start taking Synthroid. But never change the dose of your diabetes medications on your own. Instead, if you notice that your blood sugar levels increase while you’re taking Synthroid, talk with your doctor. They’ll recommend changes in your treatment to help you have healthy blood sugar levels.

Synthroid and certain heartburn drugs

Taking certain heartburn medications with Synthroid may affect how well Synthroid works.

For instance, some heartburn drugs can decrease the level of Synthroid in your body. This is because the heartburn medications change the amount of acid you have in your stomach. And Synthroid needs a certain amount of acid in your stomach in order for it to be properly absorbed. If your Synthroid level is decreased, the drug may not work as well as usual.

Examples of heartburn medications that can have this interaction with Synthroid include:

Other medications used to treat heartburn, such as calcium carbonate (Tums), may also interact with Synthroid. But Tums actually binds to Synthroid in your stomach, which stops your body from absorbing Synthroid as well as usual. To help avoid this interaction, you should take Tums at least 4 hours before or 4 hours after your Synthroid dose.

If you are taking any heartburn medications, talk with your doctor about whether the medications are safe to use with Synthroid. If you do take heartburn drugs with Synthroid, your doctor may order blood tests more often than usual to monitor your thyroid health.

Synthroid and certain drugs used for digestive issues

In addition to the heartburn medications discussed above, other medications used for digestive issues may also interact with Synthroid. This includes the medications:

  • simethicone (Gas-X)
  • sucralfate (Carafate)

These medications may decrease the amount of Synthroid in your body. And this interaction could make Synthroid less effective for you.

If you’re taking either simethicone or sucralfate, talk with your doctor about whether you should continue to use the drug with Synthroid. If you do take simethicone or sucralfate with Synthroid, your doctor may order blood tests more often than usual to monitor your thyroid health.

Synthroid and certain antidepressants

Some drugs used to treat depression may interact with Synthroid. Certain antidepressants can lower the level of Synthroid in your body, while other antidepressants can raise the level.

It’s important to note that while some antidepressants interact with Synthroid, some don’t. For example, the antidepressants escitalopram (Lexapro) and bupropion (Wellbutrin) don’t interact with Synthroid.

If you’re taking an antidepressant medication, talk with your doctor before starting Synthroid. They’ll recommend whether it’s safe for you to take the antidepressant with Synthroid. In some cases, your doctor may recommend that you switch to a different antidepressant. Or they may monitor you more often than usual while you’re using the drugs together.

Synthroid and certain antidepressants that increase levels of Synthroid

Tricyclic antidepressants (TCAs) and tetracyclic antidepressants may interact with Synthroid. If you take these drugs with Synthroid, the levels of both Synthroid and the antidepressant may be increased in your body. This means you may have an increased risk of side effects from both medications. And keep in mind that side effects of Synthroid can include serious conditions, such as irregular heartbeat.

Examples of tricyclic and tetracyclic antidepressants include:

  • amitriptyline
  • imipramine (Tofranil)
  • nortriptyline (Pamelor)
  • maprotiline
  • mirtazapine (Remeron)

Synthroid and an antidepressant that decreases levels of Synthroid

The antidepressant sertraline (Zoloft) may also interact with Synthroid. However, this drug may decrease the level of Synthroid in your body. This can make Synthroid less effective for you.

If you’re taking Synthroid along with sertraline, your doctor may increase your dosage of Synthroid. This helps your doctor to be sure that you’re getting the right amount of Synthroid for your body’s needs.

Synthroid and certain cholesterol medications

Certain medications used to treat high cholesterol may interact with Synthroid. In fact, certain cholesterol medications called bile acid sequestrants can decrease the level of Synthroid in your body. These cholesterol drugs may do this by lowering the amount of Synthroid that your body absorbs.

Examples of bile acid sequestrants include:

If you take a bile acid sequestrant, your doctor may recommend that you take Synthroid at least 4 hours before you take the bile acid sequestrant. Doing this helps to lower your risk of this drug interaction. And your doctor may also order blood tests more often than usual to monitor your thyroid health.

If you have questions about taking cholesterol medication with Synthroid, talk with your doctor.

Synthroid and certain medications that affect electrolyte levels

Certain medications that affect your electrolyte levels may interact with Synthroid. Examples of these medications include:

  • sodium polystyrene sulfonate, which is used to decrease potassium levels
  • sevelamer (Renvela), which is used to decrease phosphorous levels

These medications may decrease the amount of Synthroid that your body absorbs. This can lower the level of Synthroid in your body, making the drug less effective for you.

If you’re taking one of these drugs with Synthroid, your doctor may order certain blood tests more often than usual to monitor your thyroid health. And your doctor may adjust your dosage of Synthroid needed.

If you have questions about taking electrolyte medications while you’re taking Synthroid, talk with your doctor.

Synthroid and phenobarbital

Phenobarbital, a drug used to treat seizures, may decrease the level of Synthroid in your body. This could make Synthroid less effective for you. If you’re taking Synthroid with phenobarbital, your doctor may order blood tests more often than usual to monitor your thyroid health.

If you take phenobarbital, talk with your doctor before starting Synthroid.

Synthroid and rifampin

An antibiotic called rifampin (Rimactane) may decrease the level of Synthroid in your body. This could make Synthroid less effective for you. If you take Synthroid with rifampin, your doctor may order blood tests more often than usual to monitor your thyroid health.

If you need to take rifampin, talk with your doctor before starting Synthroid.

Synthroid and certain anticoagulant drugs

If taken with certain anticoagulants that affect your vitamin K level, Synthroid may increase the effect of the anticoagulant drug in your body. (Anticoagulants are blood thinners.)

This means that Synthroid may make certain anticoagulants, such as warfarin (Coumadin), work better than they usually do. And this may cause your blood to become too thin, which can lead to serious side effects, such as internal bleeding.

If you’re taking Synthroid with one of these anticoagulant, your doctor may decrease your dosage of the anticoagulant. This helps to lower your risk of having blood that’s too thin. Your doctor may also monitor your blood more often than usual to check that it’s not too thin.

If you’re taking an anticoagulant, talk with your doctor before using Synthroid.

Synthroid and digoxin

Synthroid may decrease the level of digoxin (Lanoxin) in your body. (Digoxin is used to treat certain heart conditions.) With this interaction, digoxin may not work as well as usual.

If you’re taking Synthroid and digoxin, your doctor will order certain tests more often than usual to monitor your digoxin level. In some cases, your doctor may increase your dosage of digoxin.

If you have any questions about taking Synthroid and digoxin together, talk with your doctor.

Synthroid and certain birth control medications

Taking certain birth control drugs that contain estrogen may decrease the level of certain hormones in your body. This interaction can affect your level of both Synthroid and the birth control medication.

Before starting Synthroid, talk with your doctor if you’re using birth control that contains estrogen. In some cases, your doctor may adjust your dosage of Synthroid. Or they may recommend that you use a different birth control option.

If you have questions about taking Synthroid and using birth control, talk with your doctor.

Synthroid and herbs and supplements

Certain mineral supplements may interact with Synthroid. However, there aren’t any herbs that have been specifically reported to interact with Synthroid. But you should still check with your doctor or pharmacist before using any herbs or supplements while you’re taking Synthroid.

Synthroid and mineral supplements

Certain mineral supplements interact with Synthroid by binding to Synthroid inside your stomach. When these supplements bind to Synthroid, your body can’t absorb Synthroid. This means that you won’t get the full amount of Synthroid that’s in your dose.

For example, the following mineral supplements are known to interact with Synthroid:

If you’re taking any of these supplements, you should take them at least 4 hours before or 4 hours after your Synthroid dose. Doing so will help make sure your dose of Synthroid is absorbed like usual.

If you have any questions about taking Synthroid with mineral supplements, talk with your doctor.

Synthroid and foods

There are some foods that you shouldn’t eat while you’re taking Synthroid. These foods may interact with the drug and change the level of Synthroid in your body.

If you have questions about eating certain foods with Synthroid, talk with your doctor.

Synthroid and coffee

Coffee may decrease the amount of Synthroid that your body absorbs. This means that if you drink coffee with your dose of Synthroid, you may not be getting the full amount of Synthroid in your dose. In this case, Synthroid may not work as well as usual for you.

If you drink coffee, talk with your doctor about how much is safe for you to drink while you’re taking Synthroid. Your doctor may recommend that you take your doses of Synthroid at different times than when you drink coffee.

Synthroid and grapefruit

Grapefruit and grapefruit juice may both interact with Synthroid, decreasing the level of Synthroid in your body. If your Synthroid level is decreased, the drug may not work as well as usual for you.

Talk with your doctor about eating grapefruit or drinking grapefruit juice while you’re taking Synthroid. Your doctor may recommend that you take your doses of Synthroid at different times than when you consume grapefruit. Or they may recommend that you avoid the fruit altogether.

Synthroid and soy

Soybeans and soybean flour may bind to Synthroid inside your stomach and stop your body from absorbing the drug properly. This can decrease your body’s level of Synthroid, making the drug less effective for you.

If you eat soybeans or soybean flour, your doctor may increase your dosage of Synthroid. Doing this will help to make sure that you have enough of the medication in your system. Your doctor may also order certain blood tests more often than usual to monitor your thyroid.

If you eat soybeans or soybean flour, talk with your doctor about whether it’s safe to eat soy while you’re taking Synthroid.

Synthroid and cottonseed meal

Synthroid may be affected by cottonseed meal. This product may bind to Synthroid inside your stomach and lower the amount of Synthroid your body absorbs. In this case, you may not have enough Synthroid in your body to treat your condition.

If you eat cottonseed meal, your doctor may increase your dosage of Synthroid. Or they may recommend that you decrease the amount of cottonseed meal in your diet. Your doctor may also check certain blood tests more often than usual to monitor your thyroid.

If you eat cottonseed meal, talk with your doctor about how safe it is to eat while you’re taking Synthroid.

Synthroid and walnuts

Walnuts can bind to Synthroid inside your stomach and lower the amount of Synthroid that your body absorbs. In this case, you may not have enough Synthroid in your body to treat your condition.

While you’re using Synthroid, your doctor may recommend that you eat fewer walnuts than usual. Or they may recommend that you eat other nuts instead. If you do eat walnuts, your doctor may order certain blood tests more often than usual to monitor your thyroid.

If you eat walnuts, talk with your doctor about how safe they are to eat while you’re taking Synthroid.

Synthroid and dietary fiber

Dietary fiber may bind to Synthroid inside your stomach, which can lower the amount of Synthroid your body absorbs. (Dietary fiber is fiber that you consume from eating certain foods.) In this case, you may not have enough Synthroid in your body to treat your condition.

While you’re taking Synthroid, your doctor may recommend that you consume dietary fiber and take your doses of Synthroid at different times. Your doctor may also recommend that you decrease your fiber intake. And they may order certain blood tests more often than usual to monitor your thyroid.

If you have questions about consuming fiber while you’re using Synthroid, talk with your doctor.

Synthroid and lab tests

Synthroid may affect certain blood tests, such as those used to check your thyroid hormone levels. In addition, a supplement called biotin may interact with labs that are used to check thyroid hormone levels. Both of these interactions are described below.

Biotin and blood tests used to monitor your thyroid

Biotin supplements may affect the blood tests used to check thyroid hormone levels. In fact, biotin may cause certain blood tests to show a high level of thyroid hormone in your body.

But this interaction is just between the blood test and the biotin supplement. It doesn’t necessarily mean that you have too much thyroid hormone in your body.

If you take biotin, your doctor may have you take less of the supplement for a few days before you have labs to check your thyroid. Or your doctor may recommend that you stop taking the biotin supplement before your labs.

Synthroid use before having blood tests to monitor your thyroid

While you’re taking Synthroid, your doctor will order certain lab tests to check your thyroid hormone levels. This is done to be sure that your levels are within a normal range.

But keep in mind that Synthroid is a man-made form of the natural thyroid hormone called T4. This means that if you take the drug before having blood work, your T4 level may be falsely elevated.

Before having blood tests to check your thyroid, ask your doctor if you should take your Synthroid doses as usual. Your doctor will recommend when you should take your Synthroid dose in relation to when your labs will be done.

Synthroid comes as tablets that are taken by mouth. You should take Synthroid according to your doctor’s or healthcare provider’s instructions.

When to take

Synthroid should be taken once daily on an empty stomach. It’s usually recommended that you take Synthroid about 30 minutes to 1 hour before breakfast. In some cases, your doctor may discuss with you how to take Synthroid at night. This may be recommended if your body absorbs Synthroid better when it’s taken at night.

In addition, Synthroid should be taken at least 4 hours before or 4 hours after you’ve taken certain medications that interact with Synthroid. For more information on drugs that may interact with Synthroid, see the “Synthroid interactions” section above.

To help make sure that you don’t miss a dose, try setting a reminder on your phone. A medication timer may be useful, too.

Taking Synthroid with food

You should take Synthroid on an empty stomach without food. If you do eat something, and you forget to take Synthroid, call your doctor or pharmacist. They’ll recommend how long you’ll need to wait after eating before you take Synthroid. This length of time may depend on the type of food you ate and whether you took any other medications.

You may need to wait 4 hours to take your Synthroid after eating. This is sometimes the case so that Synthroid doesn’t interact with foods or other medications in your body.

If you have questions about when to take Synthroid around meals, talk with your doctor or pharmacist.

Can Synthroid be crushed, split, or chewed?

Yes, you can crush, split, or chew Synthroid tablets. For example, to give the drug to children or infants, Synthroid tablets can be crushed and dissolved in 1 to 2 teaspoons (5 to 10 mL) of water. If you dissolve the drug like this, you’ll need to give the dose to the infant or child right away.

Be sure that you don’t mix crushed Synthroid into any foods. This is because Synthroid should be taken on an empty stomach.

The Food and Drug Administration (FDA) approves prescription drugs such as Synthroid to treat certain conditions. Synthroid may also be used off-label for other conditions. Off-label use is when a drug that’s approved to treat one condition is used to treat a different condition.

Synthroid for hypothyroidism

Synthroid is approved to treat hypothyroidism. With this condition, you can have symptoms such as tiredness and weight gain. You may also have muscle weakness and constipation.

With hypothyroidism, your body doesn’t make enough thyroid hormones. Thyroid hormones are very important to your health because they regulate your metabolism (all of the chemical reactions in your body). Your metabolism affects everything from your digestion to your body’s growth and repair to even your temperature.

The two main thyroid hormones in your body are called T3 and T4. Synthroid is a man-made form of T4, and it works by replacing T4 in your body. With Synthroid treatment, your T4 levels are increased to a level that’s closer to normal. Your body then uses T4 to make T3.

Synthroid is approved to treat hypothyroidism that’s congenital (present at birth) or acquired (developed later in life). And it’s approved to treat the three main types of hypothyroidism, which are:

  • Primary hypothyroidism. With this condition, your thyroid gland doesn’t produce enough thyroid hormones. Primary hypothyroidism is the most common type of hypothyroidism. It may be caused by an autoimmune disorder called Hashimoto’s thyroiditis. You may also develop primary hypothyroidism if your thyroid gland has been either treated with radiation or surgically removed. (These treatments may have occurred if you’ve had certain types of thyroid cancer.)
  • Secondary hypothyroidism. Your pituitary gland is supposed to tell your thyroid gland to make thyroid hormones. With secondary hypothyroidism, that doesn’t happen. As a result, your thyroid gland doesn’t make enough thyroid hormones. This condition may be caused by having surgery or radiation on the pituitary gland. It may also be caused by trauma to your head that affected your pituitary gland, or pituitary cancer.
  • Tertiary hypothyroidism. Your hypothalamus is supposed to tell your pituitary gland that your body needs more thyroid hormones. With tertiary hypothyroidism, this doesn’t happen. As a result, your pituitary gland doesn’t tell your thyroid gland to make enough thyroid hormones. Tertiary hypothyroidism may occur if you have cancer in your hypothalamus. The condition can also occur if you’ve had radiation or surgery on your hypothalamus in the past.

These different types of hypothyroidism have different causes. However, Synthroid works the same way no matter the cause of the condition.

Effectiveness for hypothyroidism

There aren’t any statistics to show the effectiveness of Synthroid in treating hypothyroidism. This is because there haven’t been any clinical trials that compare Synthroid or products containing levothyroxine to a placebo (treatment with no active drug). This lack of studies is due to the drug’s age and the FDA’s history of drug approval requirements (see “History of Synthroid” below).

Despite this lack of clinical trials, levothyroxine (the active drug in Synthroid) is recommended as the primary treatment for hypothyroidism. In 2014, the American Thyroid Association released guidelines showing the best treatments for hypothyroidism. These guidelines state that products that contain levothyroxine should be used as the standard treatment for hypothyroidism.

These guidelines are based on levothyroxine’s many years of successful use. The years of use have shown that levothyroxine reduces symptoms of hypothyroidism in most people. They’ve also shown that when people with hypothyroidism stop using levothyroxine, they begin having symptoms of their condition again.

In addition, levothyroxine treatment has been shown to be safe and without many serious side effects.

History of Synthroid

T4 was first recognized in the human body in 1926, and a man-made form of it (called levothyroxine) was first made in 1927. But the brand-name drug Synthroid wasn’t approved until 2002.

In 1962, new laws were created by the FDA. These laws required drugs to have their safety and effectiveness proven before they’re sold on the market. (Prior to this time, this information wasn’t required.) However, at the time, levothyroxine had already been in use for so many years that it didn’t have to go through this new process.

In 1997, the FDA required drugs made with levothyroxine to have a New Drug Application (NDA) approval. (An NDA is an application requesting that a drug be approved by the FDA.)

But again, because levothyroxine had been on the market for so long, the FDA didn’t require the drug to go through clinical trials. Instead, past studies of levothyroxine were used to show the drug’s safety and effectiveness.

Synthroid for TSH suppression

Synthroid is approved to suppress (decrease) levels of thyroid stimulating hormone (TSH). It’s approved for this use in people with a form of thyroid cancer called thyrotropin-dependent well-differentiated thyroid cancer.

TSH stimulates your thyroid to make thyroid hormones. (The two main thyroid hormones in your body are T3 and T4.) In addition, TSH is responsible for growth of cells in your thyroid gland, including growth of cancerous cells. By decreasing your TSH level, Synthroid can help decrease the growth of certain types of thyroid cancer.

Normally, when your thyroid hormone levels are low, your body makes TSH. This signals your thyroid to make more thyroid hormones. However, if you’re taking Synthroid, your thyroid hormone levels won’t be low. So your body won’t need to make TSH, and your TSH level will be lower than usual.

With a lower TSH level, growth of cancer in your thyroid may decrease. When Synthroid is used for TSH suppression, the drug is meant to be used along with surgery and radioactive iodine treatment.

Effectiveness for TSH suppression

According to the American Thyroid Association, TSH suppression is recommended in some people with high-risk forms of thyroid cancer. Levothyroxine is the recommended drug to use alongside surgery and radioactive iodine treatment.

There aren’t any clinical trials comparing TSH suppression with Synthroid to that of a placebo (no active drug). However, levothyroxine is considered the recommended drug for this use.

For information on Synthroid’s history and the lack of clinical studies of this drug, see the “History of Synthroid” section just above.

Drug limitations

The manufacturer of Synthroid has stated that the drug isn’t approved for the following uses:

Synthroid for other conditions

In addition to the uses listed above, Synthroid may be used off-label for other purposes. Off-label drug use is when a drug that’s approved for one use is used for a different one that’s not approved. And you may wonder if Synthroid is used for certain other conditions. Below is information on a use for Synthroid that’s not appropriate.

Synthroid for hyperthyroidism (not an appropriate use)

You shouldn’t use Synthroid to treat hyperthyroidism. With this condition, you have increased levels of thyroid hormones in your body. (The two main thyroid hormones are T3 and T4.)

Instead, Synthroid is FDA-approved to treat hypothyroidism, which is the opposite of hyperthyroidism. With hypothyroidism, you have low levels of thyroid hormone.

Synthroid is a man-made form of T4. It replaces or adds to your natural thyroid hormones, which helps your body to have the right amount of T4. (Your body then uses the T4 to make T3.)

Taking Synthroid for hyperthyroidism will increase the amount of thyroid hormone in your body even more than it’s already increased. This raises your risk of serious side effects, such as tremors, anxiety, and irregular heart rate.

If you have hyperthyroidism, talk with your doctor about appropriate treatment options for your condition.

Synthroid and children

In some cases, children may need to take Synthroid for certain problems affecting their thyroid. Below, we describe approved uses of Synthroid for children.

If you have questions about using Synthroid in a child, talk with your child’s doctor.

Synthroid for children with congenital hypothyroidism

From birth, some children may not be able to make enough thyroid hormones. This condition is called congenital hypothyroidism. It may occur if the child’s thyroid gland doesn’t develop properly or doesn’t work normally.

Usually, for congenital hypothyroidism, a child will take Synthroid for their entire life. This is because their body can’t make thyroid hormones on its own. Synthroid works by replacing T4, one of the main thyroid hormones. This helps the child’s T4 levels to be within a normal range. And their body then uses T4 to make T3, which is the other main thyroid hormone.

For more details about hypothyroidism, see the “Synthroid for hypothyroidism” section above.

Synthroid for children with acquired hypothyroidism

Children may also acquire (develop) hypothyroidism during their childhood. This may occur because of changes in their thyroid or pituitary. (The pituitary tells the thyroid to make thyroid hormones. So if either the pituitary or thyroid isn’t working properly, a child may develop hypothyroidism.)

Synthroid works to treat hypothyroidism by replacing T4 so that the child’s T4 levels are within a normal range. And their body then uses T4 to make T3, which is the other main thyroid hormone.

For more details about hypothyroidism, see the “Synthroid for hypothyroidism” section above.

In some cases, your doctor may recommend that you take other medications with Synthroid. Below, we describe some supplements that your doctor may recommend. However, before starting any of these supplements, be sure to talk with your doctor. They can advise whether it’s safe for you to take them.

Synthroid with vitamin D

People with hypothyroidism that’s caused by an autoimmune disease may have lower levels of vitamin D. Vitamin D plays a role in how your immune system functions. And because some thyroid conditions are related to your immune system, people with these conditions may have low vitamin D levels.

If you have hypothyroidism, your doctor may order blood tests to check your vitamin D level. And your doctor may recommend that you take a vitamin D supplement if your levels are low. For some people, increasing vitamin D level may help decrease symptoms of their thyroid condition.

Synthroid with melatonin

Melatonin is a hormone that’s made in your brain. It makes you feel sleepy when it gets dark outside.

One study showed that melatonin may increase your levels of T4. (T4 is one of the main thyroid hormones in your body, which Synthroid is used to replace or add to.) And the study showed that melatonin may also decrease feelings of anxiety and depression.

In some cases, your doctor may recommend that you take a melatonin supplement along with Synthroid. But keep in mind that melatonin may affect your thyroid hormone levels. Because of this, your doctor may check your thyroid hormone levels when you first start taking melatonin. And they may also check your levels if you change the amount of melatonin you’re taking.

Synthroid with probiotics

Sometimes hypothyroidism may cause problems with your intestines. This may happen because decreased thyroid function is linked to having too much bacteria in your small intestine. And having large amounts of bacteria in your intestine may result in certain problems, such as diarrhea.

Taking a probiotic may help to decrease harmful bacteria and increase helpful bacteria in your gut. If you’re having diarrhea or other problems with digestion while you’re taking Synthroid, talk with your doctor. They’ll advise whether you should try taking a probiotic.

Synthroid is a man-made form of a natural thyroid hormone called T4. It’s approved to treat hypothyroidism and to suppress (lower) levels of thyroid stimulating hormone (TSH). Below, we describe how Synthroid works for each of these uses.

How it works for hypothyroidism

With hypothyroidism, your body doesn’t make enough thyroid hormones. The main thyroid hormones in your body are T3 and T4. By taking Synthroid, you’re replacing your body’s T4. And your body then uses T4 to make T3.

Synthroid helps to bring your thyroid hormone levels back to a normal range. And it also helps reduce your symptoms of hypothyroidism.

How it works for TSH suppression

Synthroid is used to suppress (lower) TSH levels in people with thyrotropin-dependent well-differentiated thyroid cancer.

TSH stimulates your thyroid to make more thyroid hormones. In addition, TSH is responsible for cell growth in your thyroid gland. This includes growth of both healthy cells and cancerous cells in the gland.

Normally, when your thyroid hormone levels are low, your body makes TSH. This signals your thyroid to make more thyroid hormones. However, if you’re taking Synthroid, your thyroid hormone levels won’t be low. So your body won’t need to make TSH, and your TSH level will be lower than usual.

By decreasing your TSH level, Synthroid can help decrease the growth of certain types of thyroid cancer.

How long does it take to work?

It may take some time for you to feel any difference when you’re taking Synthroid.

The drug usually has its maximum effect in your body about 4 to 6 weeks after you start taking it. If you have thyroid-related symptoms, you may notice your symptoms are decreasing within 4 to 6 weeks after starting the drug.

After 4 to 6 weeks of treatment, your doctor will check blood tests to monitor your thyroid. And your doctor may adjust your dosage of Synthroid if needed based on your lab results.

How long does it stay in your system?

Synthroid stays in your body for about 6 to 7 days after you’ve taken a dose

However, you should try to keep thyroid hormone levels consistent in your body while you’re taking Synthroid. To do this, it’s important to take the drug as your doctor directs, and to not skip doses.

Using more than the recommended dosage of Synthroid can lead to serious side effects. Do not use more Synthroid than your doctor recommends.

Overdose symptoms

Keep in mind that Synthroid is a man-made form of the natural thyroid hormone T4. Because of this, symptoms of Synthroid overdose are also symptoms of hyperthyroidism (elevated thyroid hormone levels).

Mild symptoms of hyperthyroidism include:

More serious symptoms of a Synthroid overdose may include:

What to do in case of overdose

If you think you’ve taken too much of this drug, call your doctor. You can also call the American Association of Poison Control Centers at 800-222-1222 or use their online tool. But if your symptoms are severe, call 911 or go to the nearest emergency room right away. Don’t try to treat the overdose yourself.

Here are answers to some frequently asked questions about Synthroid.

If my TSH level is high, should my Synthroid dose be increased or decreased?

Usually, if your TSH level is high, your Synthroid dose needs to be increased. TSH, which stands for thyroid stimulating hormone, is made when your body needs more thyroid hormones.

When you have an elevated TSH level, your dose of Synthroid may need to be increased. On the other hand, if your TSH level is low, your dose of Synthroid may need to be decreased.

During Synthroid treatment, your doctor will order blood tests to check your TSH level. And for some people, their doctor will also check the levels of their thyroid hormones, T3 and T4. Based on the results of your blood tests, your doctor will adjust your dosage of Synthroid if needed.

If you have questions about whether your dosage needs adjusted, talk with your doctor.

If I stop taking Synthroid abruptly, will I have withdrawal symptoms?

Stopping Synthroid won’t make you have withdrawal symptoms. (Withdrawal symptoms may occur when you stop taking a drug that your body is dependent on. With dependence, your body needs the drug in order for you to feel normal.)

But stopping Synthroid may cause you to have symptoms of the condition you were taking the drug to treat.

For example, if you’re taking Synthroid for hypothyroidism, and you stop taking it, you may develop symptoms of hypothyroidism. This is because your symptoms were relieved when your body was getting thyroid hormone from Synthroid. But when you stop the drug, your body won’t be receiving the hormone.

If you have questions about stopping treatment with Synthroid, talk with your doctor.

Is Synthroid gluten-free?

Yes, Synthroid tablets are gluten-free. In fact, a study was done to determine whether or not Synthroid contained gluten. And the results of the study showed that there was no detectable gluten in the tablets. Because of this, Synthroid is considered to be gluten-free.

How does Synthroid relate to T3 and T4?

Synthroid is a man-made form of the natural thyroid hormone called T4. But your thyroid actually makes two main hormones: T3 and T4.

If your body doesn’t make enough thyroid hormones, your doctor may prescribe Synthroid for you. (To learn about another possible use of Synthroid, see the “Synthroid Uses” section above.) In this case, Synthroid is used to replace your natural T4. Then, your body uses T4 to make T3. (There is a man-made form of T3 called liothyronine [Cytomel], but that drug isn’t contained in Synthroid.)

Taking Synthroid can help bring your thyroid hormones to healthy levels. And the drug helps to regulate your metabolism (all of the chemical reactions in your body). Your metabolism affects everything from your digestion to your body’s growth and repair to even your temperature.

Is Synthroid a steroid?

No, Synthroid isn’t a steroid. Instead, Synthroid is a man-made form of the natural thyroid hormone called T4. Synthroid replaces or adds to the natural thyroid hormones in your body. For information on how this drug works, see the “How Synthroid works” section above.

Can Synthroid help relieve depression that’s related to hypothyroidism?

Yes, Synthroid may help relieve depression, if the depression is related to hypothyroidism. (Keep in mind that Synthroid is approved to treat hypothyroidism.)

For some people, depression may be caused by hypothyroidism. Synthroid treats hypothyroidism by increasing your thyroid hormone levels to a normal range. So if you have depression that’s related to low thyroid hormones, Synthroid may help treat your depression.

Be sure and talk with your doctor if you feel depressed. They can discuss with you treatment options that may help improve your moods.

Usually, if you’re already taking Synthroid, you’ll continue taking it during pregnancy. Some information shows that taking Synthroid during pregnancy doesn’t increase the risk of birth defects or pregnancy loss.

In fact, hypothyroidism, which Synthroid is used to treat, can actually be dangerous during pregnancy. Not having enough thyroid hormone in your body, which happens in hypothyroidism, can increase certain risks. These risks include pregnancy loss, high blood pressure during pregnancy, and premature delivery.

If you have a thyroid condition, it’s important that your doctor monitors your thyroid while you’re pregnant. In some cases, pregnancy may increase your thyroid hormone needs. In that case, you’d need to take more Synthroid than you did before pregnancy. If needed during your pregnancy, your doctor will increase your dosage of Synthroid.

If you’re pregnant or planning to become pregnant, talk to your doctor about how to manage Synthroid treatment.

In most cases, you’ll continue to take Synthroid during pregnancy if you were using the drug before becoming pregnant.

But if you’re sexually active and you or your partner can become pregnant, it’s still important to talk with your doctor about your birth control needs. They’ll recommend whether you need to use birth control while you’re taking Synthroid.

It’s important to note that Synthroid may interact with certain birth control drugs. For more information about birth control drugs that may interact with Synthroid, see the “Synthroid interactions” section above.

It’s not known whether Synthroid is safe to take while breastfeeding. Some studies have shown that the drug does pass into the breast milk of lactating women.

If you’re taking Synthroid and considering breastfeeding, talk with your doctor. They can discuss with you the risks and benefits of breastfeeding while taking this drug.

As with all medications, the cost of Synthroid can vary.

The actual price you’ll pay depends on your insurance plan, your location, and the pharmacy you use.

Your insurance plan may require you to get prior authorization before approving coverage for brand-name Synthroid. This means that your doctor and insurance company will need to communicate about your prescription before the insurance company will cover the drug. The insurance company will review the request and let you and your doctor know if your plan will cover Synthroid.

If you’re not sure if you’ll need to get prior authorization for Synthroid, contact your insurance company.

Financial and insurance assistance

If you need financial support to pay for Synthroid, or if you need help understanding your insurance coverage, help is available.

AbbVie, Inc., the manufacturer of Synthroid, offers ways to help lower the cost of this drug. For example, the company offers both a copay card and coupons. For more information about these offers and to find out if you’re eligible for support, visit the program website.

Generic version

Synthroid is available in a generic form called levothyroxine. A generic drug is an exact copy of the active drug in a brand-name medication. The generic is considered to be as safe and effective as the original drug. And generics tend to cost less than brand-name drugs.

Switching between brand-name Synthroid and its generic forms

It’s important to talk with your doctor before switching from brand-name Synthroid to any of its generic forms. This is because doses of the different versions of this drug may not be equivalent. For more information, see the “Synthroid generic” section above.

To find out how the cost of levothyroxine compares to the cost of Synthroid, visit GoodRx.com. Again, the cost you find on GoodRx.com is what you may pay without insurance. The actual price you’ll pay depends on your insurance plan, your location, and the pharmacy you use.

If your doctor has prescribed Synthroid and you’re interested in using levothyroxine instead, talk with your doctor. They may have a preference for one version or the other. You’ll also need to check your insurance plan, as it may only cover one or the other.

This drug comes with several precautions.

FDA warning: Not for use to treat obesity or as a weight loss aid

This drug has a boxed warning. This is the most serious warning from the Food and Drug Administration (FDA). A boxed warning alerts doctors and patients about drug effects that may be dangerous.

Synthroid shouldn’t be used to treat obesity or to help with weight loss. If your thyroid function is normal, Synthroid won’t help you lose weight. In fact, using the drug at typical dosages in people without certain thyroid problems won’t lead to weight loss. And using doses larger than those recommended by the manufacturer may cause serious side effects. These side effects can include confusion, seizures, coma, and even death.

In addition, taking Synthroid along with drugs used for weight loss may cause very serious or even life threatening side effects.

Because of these possible risks, Synthroid should only be taken as prescribed by your doctor.

Other precautions

Before taking Synthroid, talk with your doctor about your health history. Synthroid may not be right for you if you have certain medical conditions or other factors affecting your health. These include:

  • Heart conditions, such as atrial fibrillation (A-fib). Synthroid may cause increased heart rate and irregular heartbeat patterns to occur. If you already have a heart condition, such as A-fib, Synthroid may worsen your condition. If you have certain heart conditions, your doctor may start you on a dosage of Synthroid that’s lower than usual. This allows your doctor to see how the drug affects you. And your doctor may also monitor your heart more often than usual to be sure your heart condition isn’t worsening. Before starting Synthroid, talk with your doctor about any heart conditions you have.
  • Myxedema coma. Synthroid shouldn’t be used to treat myxedema coma (severe hypothyroidism that may lead to decreased brain function). This condition is a medical emergency. It usually needs to be treated with thyroid hormones given by intravenous (IV) injection. (With an IV injection, the drug is injected into one of your veins.) If you have or develop myxedema coma, you need to go to the hospital right away. Do not attempt to treat the condition by taking Synthroid.
  • Adrenal gland problems. Synthroid can decrease your level of adrenal hormones, such as cortisol. If you have certain adrenal gland problems, such as adrenal insufficiency, your adrenals already don’t make enough hormones. If you take Synthroid, your adrenal hormone levels may become very low. If you have adrenal insufficiency, your doctor will give you adrenal hormones before you start Synthroid. You shouldn’t take Synthroid until your adrenal hormones are within a normal range. Once the levels are normal, you can start taking Synthroid. Before starting Synthroid, tell your doctor about any adrenal gland problems you may have.
  • Diabetes. If you have diabetes and you’re taking certain diabetes drugs, Synthroid may cause your blood sugar level to increase. This is because if taken with Synthroid, your diabetes medications may not work as well as usual to manage your blood sugar level. If you have diabetes, tell your doctor before starting Synthroid. They’ll likely monitor your blood sugar levels more often than usual. This helps your doctor to ensure that your blood sugar levels are adequately managed.
  • Blood clotting problems. If you’re taking certain medications called anticoagulants, tell your doctor before starting Synthroid. (Anticoagulants are blood thinners.) Synthroid may make anticoagulants work better than usual, which can make your blood too thin. This can cause serious side effects, such as internal bleeding. If you’re taking Synthroid with an anticoagulant, your doctor may monitor your blood more often than usual . This helps your doctor to know whether they need to lower the dose of your anticoagulant. Before starting Synthroid, talk with your doctor about any blood clotting disorders you have or any medications you take for blood clots.
  • Allergic reaction. If you’ve had an allergic reaction to Synthroid or to any of its ingredients, you shouldn’t take Synthroid. Instead, talk with your doctor what other medications are safe for you to take.
  • Pregnancy. Usually, Synthroid can be taken during pregnancy. In fact, taking Synthroid during pregnancy doesn’t increase the risk of birth defects or miscarriage. In some cases, your doctor will increase your Synthroid dosage while you’re pregnant. For more information, please see the “Synthroid and pregnancy” section above.
  • Breastfeeding. It’s not known if Synthroid is safe to take while breastfeeding. If you’re breastfeeding, talk to your doctor before starting Synthroid. For more information, please see the “Synthroid and breastfeeding” section above.

Note: For more information about the potential negative effects of Synthroid, see the “Synthroid side effects” section above.

When you get Synthroid from the pharmacy, the pharmacist will add an expiration date to the label on the bottle. This date is typically 1 year from the date they dispensed the medication.

The expiration date helps guarantee that the medication is effective during this time. The current stance of the Food and Drug Administration (FDA) is to avoid using expired medications. If you have unused medication that has gone past the expiration date, talk to your pharmacist about whether you might still be able to use it.

Storage

How long a medication remains good can depend on many factors, including how and where you store the medication.

Synthroid tablets should be stored at room temperature, between 59°F and 86°F (15°C and 30°C). The drug should be kept in a tightly sealed container away from light. Avoid storing this medication in areas where it could get damp or wet, such as in bathrooms.

Disposal

If you no longer need to take Synthroid and have leftover medication, it’s important to dispose of it safely. This helps prevent others, including children and pets, from taking the drug by accident. It also helps keep the drug from harming the environment.

This article provides several useful tips on medication disposal. You can also ask your pharmacist for information on how to dispose of your medication.

The following information is provided for clinicians and other healthcare professionals.

Indications

Synthroid is indicated for the treatment of primary, secondary, and tertiary hypothyroidism that is either congenital or acquired. In addition, the drug is indicated for use in combination with surgery and radioiodine treatment for thyroid stimulating hormone (TSH) suppression as a treatment for thyrotropin-dependent well-differentiated thyroid cancer.

Administration

Synthroid is a tablet that is taken orally, typically once daily. It should be taken on an empty stomach, about 30 minutes to 1 hour before breakfast.

Mechanism of action

Synthroid is a synthetic form of the natural thyroid hormone T4.

It works to treat hypothyroidism by supplementing natural T4 that is not being produced by the body due to issues with the thyroid, pituitary, or hypothalamus. T4 can then be converted inside the body into T3, which is generally responsible for the physical effects of thyroid hormones, including metabolism, temperature regulation, and digestion.

Synthroid may also be used along with surgery and radioiodine therapy for TSH suppression. In this case, the drug is being used to treat thyrotropin-dependent well-differentiated thyroid cancer. Synthroid works by supplementing T4 levels, which lowers physiologic production of TSH. Decreased TSH levels cause decreased tumor growth in thyrotropin-dependent well-differentiated thyroid cancer.

Pharmacokinetics and metabolism

Synthroid is mostly absorbed in the small intestine. The drug is absorbed at a rate of about 40% to 80%. A fasting state increases Synthroid absorption, which is why it is recommended that Synthroid be taken an empty stomach. Foods such as soybeans, grapefruit, and those high in fiber may affect the absorption of Synthroid. See the “Synthroid interactions” section above for more information about food interactions with this drug.

Other drugs may also affect absorption of Synthroid. For more information on drug interactions, see the “Synthroid interactions” section above.

Synthroid is highly protein bound (99%). The half-life of Synthroid is about 6 to 7 days, on average. However, the drug lasts longer in the body when taken by people with hypothyroidism. The half-life in people with hypothyroidism is usually between 9 and 10 days. In comparison, the half-life in people with hyperthyroidism is between 3 and 4 days.

Synthroid is degraded mostly in the liver, but it is also degraded in the kidneys and other areas of the body by a process called deiodination. With deiodination, iodine is removed from the thyroid hormone, causing it to become inactive. The majority (80%) of T4 is deiodinated into T3, which is then deiodinated again. Thyroid hormones may also be broken down in the liver by glucuronidation.

Synthroid is mostly excreted via the kidneys. About 20% of the drug is excreted through the feces.

Contraindications

Synthroid is contraindicated in people with adrenal insufficiency that is untreated. Synthroid may decrease adrenal hormones, which may cause people with preexisting adrenal insufficiency to go into adrenal crisis. Always treat adrenal insufficiency prior to starting patients on Synthroid.

Storage

Synthroid tablets should be stored at room temperature, between 59°F to 86°F (15°C and 30°C). The drug should be kept away from moisture and light.

Disclaimer: Medical News Today has made every effort to make certain that all information is factually correct, comprehensive, and up-to-date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or other healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.