Synthroid (levothyroxine) is a brand-name drug that’s prescribed for hypothyroidism and thyroid cancer in adults and children. Synthroid comes as an oral tablet that’s typically taken once per day. The dosage varies depending on the condition the drug is used to treat.

Synthroid is approved by the Food and Drug Administration (FDA) to:

Synthroid belongs to a drug class called thyroid hormones. The drug is available in a generic version.

Keep reading for specific information about the dosage of Synthroid, including its strengths and how to take the medication. For a comprehensive look at Synthroid, see this article.

Note: This article describes typical dosages for Synthroid provided by the drug’s manufacturer. When taking Synthroid, always follow the dosage prescribed by your doctor.

* For treating thyroid cancer, Synthroid is taken along with radioactive iodine treatment and surgery.

Synthroid is approved to treat hypothyroidism and a type of thyroid cancer. Below is important information about Synthroid dosage for these uses.

Synthroid form

Synthroid comes as oral tablets.

Synthroid strengths

Synthroid oral tablets are available in the following strengths: 25 micrograms (mcg), 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg, 175 mcg, 200 mcg, and 300 mcg.

Note that Synthroid is dosed in micrograms. This is different from most other medications, which are typically dosed in milligrams (mg).

Typical dosages

The dosage range for Synthroid depends on the condition you’re taking the drug to treat and how effective the drug is for you. 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. Your doctor will ultimately prescribe the smallest dosage that provides the desired effect.

For example, a recommended dosage of Synthroid for treating hypothyroidism is 1.6 mcg of drug per kilogram (mcg/kg) of body weight, once per day. Since this dosage varies based on body weight, your dosage may be different from other people’s.

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

Dosage for hypothyroidism in adults

For treating hypothyroidism in adults, the typical starting Synthroid dosage is 1.6 micrograms per kilogram (mcg/kg) of body weight once per day. (For information on children’s dosages of Synthroid for treating hypothyroidism, see the “Children’s dosage” section below.)

For example, if you weigh 80 kg* (about 176 lb), your starting Synthroid dosage would be 125 mcg per day. This is rounded down from about 128 mcg. Your doctor will prescribe the closest available strength of Synthroid tablets.

Certain factors may affect the dosage that your doctor recommends before you start taking Synthroid. (See the “Factors that can affect your dosage” section below.) Your doctor may also adjust your dosage during treatment if needed.

* One kg is equal to about 2.2 pounds (lb).

Dosage for TSH suppression

Synthroid is approved to treat a type of thyroid cancer in adults and children. The drug does this by suppressing (decreasing) levels of thyroid-stimulating hormone (TSH).

For this use, the manufacturer of Synthroid does not recommend specific dosages. Your Synthroid dosage for TSH suppression will depend on how severe your thyroid cancer is as well as your treatment goals. Your doctor will determine a dose of Synthroid for TSH suppression that’s best for you. They may also adjust your dosage during treatment if needed.

What’s considered a high dose of Synthroid?

For treating hypothyroidism, dosages above 200 mcg per day are rarely needed, according to Synthroid’s manufacturer. (The manufacturer provides no dosage guidance for TSH suppression.)

There’s no maximum dosage of Synthroid provided by its manufacturer. So, for example, 88 mcg of Synthroid per day to treat hypothyroidism isn’t considered a high dose.

Long-term use

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.

Children’s dosage

Below is a dosage table for using Synthroid to treat hypothyroidism in children. As in adults, children’s dosages are given as micrograms per kilogram of body weight (mcg/kg).

AgeDaily Synthroid dosage
0–3 months10–15 mcg/kg
3–6 months8–10 mcg/kg
6–12 months6–8 mcg/kg
1–5 years5–6 mcg/kg
6–12 years4–5 mcg/kg
13 years or older, but still undergoing puberty or still growing2–3 mcg/kg
adolescent, and puberty and growth are completed1.6 mcg/kg

For example, the typical Synthroid dosage for a 5-year-old who weighs 25 kg (about 55 lb) would be 125–150 mcg per day.

In adolescents who’ve completed puberty and growth, doctors may adjust their dosage during treatment as needed.

Synthroid is approved to treat a type of thyroid cancer in children as well as adults. For information about Synthroid’s dosage for this use, see the “Dosage for TSH suppression” section above.

Below are answers to a few important questions about Synthroid dosages.

Is there a dosage calculator for Synthroid?

There is a dosage calculator for Synthroid, which is for use by doctors and other healthcare professionals. A dosage calculator can be helpful because thyroid hormone drugs such as Synthroid require careful monitoring and precise dosages. A dosage calculator helps make sure that the dose your doctor prescribes is safe for you.

If you have questions about how your Synthroid dosage will be calculated, talk with your doctor or pharmacist.

Can lowering the dosage of Synthroid cause side effects?

It’s unlikely that lowering Synthroid’s dosage causes side effects. But lowering your Synthroid dosage could cause or worsen symptoms of the condition you’re taking the drug to treat. This is because your body may no longer get enough thyroid hormone to help prevent or relieve your symptoms.

If you have other questions about Synthroid dose adjustments and side effects, talk with your doctor or pharmacist.

If you use more Synthroid than your doctor prescribes, you may develop serious side effects.

Synthroid is a synthetic (artificial) thyroid hormone. Your body also naturally makes thyroid hormones. For this reason, Synthroid overdose can cause the same symptoms as hyperthyroidism (high thyroid hormone levels).

It’s important that you don’t use more Synthroid than your doctor recommends.

Symptoms of an overdose

Mild overdose symptoms of Synthroid can include:

A more serious Synthroid overdose may cause additional symptoms, including:

In rare cases, Synthroid overdose may be fatal.

If you take more than the recommended amount of Synthroid

Call your doctor right away if you believe you’ve taken too much Synthroid. Another option is to call America’s Poison Centers at 800-222-1222 or use its online tool. If you have severe symptoms, immediately call 911 or your local emergency number, or go to the nearest emergency room.

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 and body weight
  • any side effects you have
  • your thyroid hormone levels

Other medical conditions you have can also affect your Synthroid dosage.

Dosage adjustments

Your doctor may need to adjust your dosage while your condition is being treated with Synthroid.

Dosage adjustments for hypothyroidism

While you’re taking Synthroid for hypothyroidism, your doctor will monitor your thyroid hormone levels with blood tests. If your Synthroid dosage needs to be adjusted, your doctor will likely adjust it in amounts of 12.5–25 mcg.

Your doctor will likely make dosage adjustments every 4–6 weeks until your thyroid hormone levels are within the target range. This period between dosage adjustments can vary depending on age or whether you have certain conditions. For example, children may have their dosage adjusted every 2 weeks after the start of treatment. Adults with certain heart conditions may have their dosage adjusted every 6–8 weeks.

Your doctor may continue to adjust your Synthroid dosage until your thyroid hormone levels are in a normal range.

Dosage adjustments for TSH suppression

Synthroid is approved to treat a type of thyroid cancer in certain situations. The drug does this by suppressing (decreasing) levels of thyroid-stimulating hormone (TSH).

If you’re taking Synthroid for this purpose, your doctor will monitor your TSH levels. They may adjust your Synthroid dosage depending on these levels.

Synthroid comes as oral tablets, which you take by swallowing. You should take Synthroid according to your doctor’s instructions. If you have trouble swallowing tablets, you can crush a Synthroid tablet and dissolve it in a small amount of water. See the instructions in the Synthroid prescribing information for how to do this correctly.

Synthroid should be taken on an empty stomach, once daily. Specifically, it should be taken 30 minutes to 1 hour before or after eating.

Usually, it’s recommended to take the drug 30 minutes to 1 hour before breakfast. But your doctor may also discuss how to take Synthroid at night. In some cases, the body can absorb the drug better if taken at night.

Note that you should take Synthroid at least 4 hours before or 4 hours after certain medications that it can interact with. For more information about drug interactions with Synthroid, refer to this article.

If you forget a dose of Synthroid, call your doctor’s office. They can recommend the best time for you to take your next dose.

Missing a dose of Synthroid could cause your thyroid hormone levels to decrease. This could lead to side effects. For this reason, it’s important to take your Synthroid dose at about the same time every day.

To help make sure that you don’t miss a dose, try using a medication reminder. This can include setting an alarm or timer on your phone or downloading a reminder app. A kitchen timer can work, too.

Stopping Synthroid doesn’t cause withdrawal. With withdrawal, you have unpleasant symptoms when you stop taking a drug that your body has become dependent on. (With drug dependence, your body needs the drug to feel like it usually does.) But you shouldn’t stop taking Synthroid unless your doctor recommends it. Stopping the drug can cause symptoms of the condition you’re taking it for to come back or get worse.

Stopping Synthroid can also affect other medical conditions you have or other medications you take. For example, the dosage of certain diabetes drugs may need to be adjusted after stopping Synthroid treatment. This is because Synthroid can make certain diabetes drugs less effective.

Talk with your doctor if you have questions about ending your treatment with Synthroid.

The dosages in this article are typical dosages provided by the drug’s manufacturer. If your doctor recommends Synthroid for you, they will prescribe the dosage that’s right for you. Always follow the dosage that your doctor prescribes for you.

As with any drug, never change your dosage of Synthroid without your doctor’s approval. If you have questions about the dosage of Synthroid that’s right for you, talk with your doctor.

Besides learning about dosage, you may want other information about Synthroid. These additional articles might be helpful:

  • More about Synthroid: For information about other aspects of Synthroid, refer to this article.
  • Side effects: To learn about the side effects of Synthroid, see this article. You can also look at the Synthroid prescribing information.
  • Drug comparison: To find out how Synthroid compares with levothyroxine, the generic version of the drug, read this article.
  • Cost: If you’d like to learn about Synthroid and cost, see this article.
  • Interactions: For details about what Synthroid interacts with, see this article.
  • Details about hypothyroidism and thyroid cancer: For details about hypothyroidism, see our list of hypothyroid articles. For more on thyroid cancer, refer to our cancer hub.

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 another 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.