Synthroid (levothyroxine) is a prescription brand-name medication. It’s approved by the Food and Drug Administration (FDA) to treat:

Hypothyroidism causes your body to not make enough thyroid hormones. Your body needs thyroid hormones for your metabolism to work correctly. Metabolism affects everything from your digestion and temperature to your body’s ability to repair itself after injury.

With thyrotropin-dependent well-differentiated thyroid cancer, a thyroid hormone called thyroid stimulating hormone (TSH) can cause cancerous thyroid cells to grow. Synthroid helps lower levels of TSH, which may help stop the cancer from growing.

Here are some fast facts on Synthroid:

  • Active ingredient: levothyroxine, which is available as a generic
  • Drug class: hormone
  • Drug form: oral tablet

As with other drugs, Synthroid can cause side effects. Read on to learn about potential common, mild, and serious side effects of the medication. For a general overview of Synthroid, including details about its uses, see this article.

Mild side effects can occur with Synthroid use. This list doesn’t include all possible mild side effects of the drug. For more information, you can refer to Synthroid’s prescribing information.

Mild side effects that have been reported with Synthroid include:

These side effects may be temporary, lasting a few days or weeks. But if the side effects last longer than that, bother you, or become severe, be sure to talk with your doctor or pharmacist.

Note: After the Food and Drug Administration (FDA) approves a drug, it tracks side effects of the medication. If you develop a side effect while taking Synthroid and want to tell the FDA about it, visit MedWatch.

* For more information on this side effect, see “Side effect specifics” below.

Synthroid may cause serious side effects. The list below may not include all possible serious side effects of the drug. For more information, you can refer to Synthroid’s prescribing information.

If you develop serious side effects while taking Synthroid, call your doctor right away. If the side effects seem life threatening or you think you’re having a medical emergency, immediately call 911 or your local emergency number.

Serious side effects that have been reported and their symptoms can include:

  • Decreased bone mineral density. Symptoms can include:
    • decreased height
    • bones that break easily
  • Heart problems.*
  • Allergic reaction.*†

* For more information on this side effect, see “Side effect specifics” below.
† An allergic reaction is possible after using Synthroid. But this side effect wasn’t reported in clinical studies.

Long-term side effects

Most side effects of Synthroid are not serious and usually go away on their own. However, some side effects can be long-term. These include decreased bone mineral density (BMD). Females* who have gone through menopause may be at a higher risk for decreased BMD.†

If you have questions about long-term side effects from Synthroid, talk with your doctor or pharmacist.

* Sex and gender exist on spectrums. Use of the term “female” in this article refers to sex assigned at birth.
† For more information about the risk of decreased BMD, see the “Synthroid precautions” section below.

Side effects of taking too much Synthroid

Side effects of taking too much Synthroid are similar to the side effects of hyperthyroidism (high levels of thyroid hormones). This is because Synthroid is a synthetic version of thyroxine, a thyroid hormone that your body makes naturally.

Side effects of taking too much Synthroid, which are also symptoms of hyperthyroidism, can include:

Remember to take Synthroid exactly as your doctor prescribes. You shouldn’t take more than the dose recommended by your doctor. This can help you avoid side effects from taking too much Synthroid.

Synthroid may cause several side effects. Here are some frequently asked questions about the drug’s side effects and their answers.

Can Synthroid cause weight gain?

Weight gain isn’t a known side effect of Synthroid. Weight gain wasn’t reported by people taking the drug in clinical trials.

Weight gain is a known symptom of hypothyroidism (low levels of thyroid hormones), which Synthroid is used to treat. If you have low levels of thyroid hormones in your body, your metabolism may be slowed down. This may lead to weight gain.

If you notice weight gain while taking Synthroid, talk with your doctor. They may want to order blood tests to check your thyroid hormone levels. Your doctor may adjust your Synthroid dose based on these tests.

What are Synthroid’s side effects in older adults?

Synthroid’s side effects in older adults are the same as those seen in other adults. For more information, see the mild and serious side effects sections above.

If you have additional questions about using Synthroid in older adults, talk with your doctor or pharmacist.

Are the side effects of Synthroid and its generic version different?

No, side effects of Synthroid and its generic version, levothyroxine, aren’t expected to be different. This is because Synthroid contains the active drug levothyroxine.

For more information on how Synthroid and levothyroxine compare, see this article.

Is joint pain a side effect of Synthroid?

No, joint pain isn’t a reported side effect of Synthroid, according to clinical trials. But joint pain has been reported as a symptom of an allergic reaction to Synthroid. (For more about allergic reactions, see “Side effect specifics” below.)

If you’re concerned about joint pain, talk with your doctor or pharmacist.

Can Synthroid cause dry mouth?

It isn’t likely. People taking Synthroid didn’t report dry mouth as a side effect in clinical trials.

But dry mouth can be a symptom of hypothyroidism, which Synthroid is used to treat. With treatment, dry mouth will likely go away.

If you have additional questions about dry mouth, talk with your doctor or pharmacist.

If I stop using Synthroid, could I have side effects of withdrawal?

No, stopping the use of Synthroid shouldn’t cause withdrawal side effects. But it could cause symptoms of the condition the drug is treating to return. For example, if you use Synthroid to treat hypothyroidism and stop taking the medication, symptoms of hypothyroidism may return.

If you have questions about stopping treatment with Synthroid, talk with your doctor. They can advise you on how to stop taking the drug safely.

Learn more about some of the side effects that Synthroid may cause.

Hair loss

It’s possible to experience hair loss while taking Synthroid. This was a side effect reported by people taking the drug in clinical trials. But hair loss rarely occurs with Synthroid.

Partial hair loss can occur during the first few months of taking Synthroid. This happens only in rare cases, and it usually improves on its own with time.

It’s also important to note that having thyroid hormone levels that are too high or too low can affect your hair growth.

What you can do

Talk with your doctor if you experience hair loss while taking Synthroid. They may be able to recommend ways to help this side effect. Your doctor may also order blood tests to check your thyroid hormone levels to see if they’re too high or too low. They may adjust your Synthroid dose based on these tests.

Rash

It’s possible to develop a skin rash while taking Synthroid. But it isn’t known how often this has happened in people taking the drug in clinical trials.

What you can do

If you develop a rash while taking Synthroid, talk with your doctor. Although this side effect isn’t serious in most people, it could be a symptom of an allergic reaction. Your doctor will typically want to see you if you develop a rash while taking Synthroid. They can help determine the cause.

Heart problems

The use of Synthroid may cause heart problems, such as arrhythmia and heart palpitations. Arrhythmia is an irregular heart rate or rhythm. Heart palpitations refers to the feeling of skipped or extra heartbeats. These side effects aren’t common with Synthroid.

Symptoms of heart problems from taking Synthroid can include:

Rarely, a heart attack can also be a side effect of taking Synthroid. Symptoms of a heart attack include chest pain and shortness of breath.

Some people may be at higher risk for heart problems as a side effect of Synthroid. This includes older adults and people who already have cardiovascular (heart and blood vessel) disease. To learn more, see the “Precautions for Synthroid” section below.

What you can do

Talk with your doctor right away if you experience any of the symptoms mentioned above. If you develop symptoms of heart problems, your doctor will likely reduce your Synthroid dose. Or, they may have you stop taking the medication for a week before lowering your dose. After the dose adjustment, your doctor may also want to monitor you more closely than usual. The goal is to make sure you don’t continue to experience heart problems.

Weight loss

Thyroid hormones, including Synthroid, shouldn’t be used to treat obesity or weight loss. In fact, Synthroid has a boxed warning regarding this. Boxed warnings are serious warnings from the Food and Drug Administration (FDA).

Taking doses of thyroid hormones that are higher than recommended can lead to life-threatening side effects, such as coma, and in rare cases, death.

In addition to the boxed warning, weight loss has been reported as a side effect of Synthroid. It isn’t known how often this occurred in clinical trials.

Weight loss isn’t unexpected, based on how Synthroid works. The drug is a synthetic version of thyroxine, a thyroid hormone your body makes naturally. Thyroxine plays a role in your metabolism, which affects digestion. (“Metabolism” refers to all the chemical reactions that happen in your body.)

What you can do

If you experience weight loss while taking Synthroid, talk with your doctor. They may want to order blood tests to check your thyroid hormone levels. Your doctor may adjust your Synthroid dose based on these tests.

Allergic reaction

As with most drugs, Synthroid can cause an allergic reaction in some people. But this side effect wasn’t reported in clinical studies.

Symptoms can be mild or serious and can include:

  • rash
  • itching
  • joint pain
  • flushing (temporary warmth, redness, or deepening of skin color)
  • swelling under your skin, typically in your lips, eyelids, feet, or hands
  • swelling of your mouth, tongue, or throat, which can make it hard to breathe

Synthroid is a synthetic version of thyroxine, a thyroid hormone your body makes naturally. There aren’t known cases of people having a reaction to levothyroxine (the active drug in Synthroid). Instead, it’s thought that inactive ingredients, such as color additives, cause allergic reactions to the drug. If you have questions about the ingredients in Synthroid tablets, talk with your doctor or pharmacist.

What you can do

For mild symptoms of an allergic reaction, call your doctor right away. They may recommend ways to ease your symptoms and determine whether you should keep taking Synthroid. But if your symptoms are serious and you think you’re having a medical emergency, immediately call 911 or your local emergency number.

Synthroid has a boxed warning about how the drug shouldn’t be used to treat obesity or weight loss. This is a serious warning from the Food and Drug Administration (FDA). For details, see “Weight loss” in the “Side effect specifics” section above.

Other precautions

Be sure to talk with your doctor about your health history before you take Synthroid. This drug may not be the right treatment for you if you have certain medical conditions or other factors that affect your health. The conditions and factors to consider include:

Heart conditions. Taking Synthroid may cause heart-related side effects, such as heart palpitations or an increased heart rate. If you already have a heart condition, such as atrial fibrillation, you may be at a higher risk for these side effects. And taking Synthroid could make your heart condition worse. If you have a heart condition, your doctor may start you on a low dose and monitor you more closely than usual.

Adrenal gland problems. Treatment with Synthroid can lower levels of your adrenal hormones, such as cortisol. People with adrenal gland problems, such as adrenal insufficiency, may be at a higher risk for this side effect. You shouldn’t start taking Synthroid unless your adrenal hormone levels are in a normal range. Be sure your doctor is aware of any adrenal gland problems you have before you start taking Synthroid.

Diabetes. Synthroid may cause your blood sugar level to rise if you have diabetes and take certain medications for it. This is because Synthroid may cause some diabetes medications to work less well than usual. Your doctor may monitor your blood sugar more closely than usual if you have diabetes and begin taking Synthroid.

Allergic reaction. You shouldn’t take Synthroid If you’ve ever had an allergic reaction to Synthroid or any of its ingredients. Talk with your doctor about which other treatments are better choices for you.

Problems with blood clotting. Certain medications called blood thinners might work better than usual if used together with Synthroid. But this can also lead to serious side effects, such as excessive bleeding. Your doctor may monitor you more closely than usual if you take Synthroid with a blood thinner. They may also lower the dose of your blood thinner. Before you start taking Synthroid, it’s important you make your doctor aware of any blood clotting problems you may have.

Being an older adult. Taking Synthroid may cause heart-related side effects, such as heart palpitations or an increased heart rate. If you’re an older adult, you may be at a higher risk for these side effects. So your doctor may start you on a low dose and monitor you more closely than usual.

Having gone through menopause. Synthroid may cause decreased bone mineral density (BMD). Females* who have gone through menopause may be at a higher risk for this side effect. Before you start taking Synthroid, tell your doctor if you’ve gone through menopause. They may monitor your BMD throughout your treatment.

* Sex and gender exist on spectrums. Use of the term “female” in this article refers to sex assigned at birth.

Alcohol use with Synthroid

There aren’t any known interactions between consuming alcohol and taking Synthroid.

But drinking alcohol may affect your thyroid hormone levels.

Your doctor may change your Synthroid dose based on your hormone levels. So it’s important for them to know whether you consume alcohol and how much you drink. This helps them choose a Synthroid dose that works best for you.

Before you start taking Synthroid, be sure your doctor is aware of whether you consume alcohol.

Pregnancy and breastfeeding while taking Synthroid

Here’s some information on pregnancy and breastfeeding while taking Synthroid.

Pregnancy. It’s generally safe to use Synthroid while pregnant. Taking Synthroid doesn’t appear to increase the risks of birth defects or pregnancy loss. But if you become pregnant during your treatment, your doctor may need to increase your dose of Synthroid. This is because pregnancy can sometimes increase the need for thyroid hormones.

Keep in mind that untreated hypothyroidism can be dangerous during pregnancy.

Breastfeeding. It isn’t known whether it’s safe to take Synthroid while breastfeeding. The drug is believed pass into human breast milk. But it isn’t known what effects, if any, this could have on a breastfed child. Talk with your doctor about the risks and benefits of treatments for your condition while breastfeeding.

Side effects from Synthroid can occur but aren’t common. Most mild side effects of the drug go away with time and don’t require medical attention. Taking the drug exactly as prescribed by your doctor should lower your risk for side effects of Synthroid. Most of the drug’s side effects are caused by having high levels of thyroid hormones in your body.

In rare cases, Synthroid can cause serious side effects. You should talk with your doctor if you experience symptoms of:

You should also talk with your doctor if you become pregnant while taking Synthroid. If you become pregnant, your doctor may need to increase your dose of the drug.

In addition, it’s important to note that Synthroid should not be used to treat obesity or weight loss.* Doing so can lead to life-threatening side effects, such as coma, and in rare cases, death.

If you’d like to learn more about Synthroid, talk with your doctor or pharmacist. They can help answer any questions you have about side effects of taking the drug.

* Synthroid has a boxed warning regarding this. A boxed warning is the most serious warning from the Food and Drug Administration (FDA). To learn more, see “Weight loss” in the “Side effect specifics” section above.

Further reading

Besides talking with your doctor, you can do some research on your own. These articles might help:

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.