Key takeaways

  • More common side effects of Synthroid include irregular heartbeat, shortness of breath, headache, muscle weakness or spasms, and increased appetite, which may lead to weight gain.
  • Synthroid should not be used for weight loss or to treat obesity due to the risk of serious and potentially life-threatening side effects. Be sure to discuss any concerns and health history with a doctor before starting the medication.
  • If side effects from Synthroid become difficult to tolerate, talk with your doctor about coping tips or other treatment options.

If you have certain thyroid conditions, your doctor might suggest Synthroid as a treatment option for you. It’s a prescription drug used to treat the following in adults and children of all ages:

Like other drugs, Synthroid can cause mild to serious side effects (also called adverse effects). Keep reading to learn more. For more information about Synthroid, including details about its uses, see this in-depth article.

Synthroid has a boxed warning. A boxed warning is the most serious warning from the Food and Drug Administration (FDA). For details, see the “Boxed warning for Synthroid” section.

Some people may experience mild to serious side effects during their Synthroid treatment. It’s important to note that most Synthroid side effects are caused by high thyroid hormone levels. Small changes in your dosage of this drug can make big differences in your thyroid hormone levels. And this can lead to side effects.

Examples of Synthroid’s commonly reported side effects include:

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Examples of mild side effects that have been reported with Synthroid include:

In most cases, these side effects should be temporary. And some may be easily managed. But if you have symptoms that are ongoing or bother you, talk with your doctor or pharmacist. And do not stop taking Synthroid unless your doctor recommends it.

Synthroid may cause mild side effects other than those listed above. See the drug’s prescribing information for details.

For more information about some of these side effects, see “Tips for managing side effects.”

Although most side effects of Synthroid are mild, it’s possible to have serious side effects from this drug.

Serious side effects that have been reported with Synthroid include:

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.

Note: After the Food and Drug Administration (FDA) approves a drug, it tracks and reviews side effects of the medication. Sharing your experience helps make medication safer for everyone by giving doctors and researchers more information about how the drug works in real life. If you develop a side effect while taking Synthroid and want to tell the FDA about it, visit MedWatch or call 800-FDA-1088.

Allergic reaction

For some people, Synthroid can cause an allergic reaction. But it’s not clear whether this side effect occurred in studies.

In general, symptoms of an allergic reaction can be mild or serious.

What might help

If you have mild symptoms of an allergic reaction, such as a mild rash, call your doctor right away. They may suggest a treatment to manage your symptoms and can determine whether you should continue Synthroid treatment.

If you have symptoms of a severe allergic reaction, such as swelling or difficulty breathing, call 911 or your local emergency number right away. These symptoms could be life threatening and require immediate medical care.

If your doctor confirms you’ve had a serious allergic reaction to Synthroid, they may have you switch to a different treatment.

Synthroid is used to treat hypothyroidism (underactive thyroid) and a certain kind of thyroid cancer in children of all ages.

Most side effects seen in children taking Synthroid are similar to those seen in adults taking the same drug. But a few side effects have been reported in children only. These include:

  • increased pressure inside the skull
  • hip dislocation
  • early closure of fontanelles, which may affect head shape and brain growth
  • early closure of growth plates (areas where bones are still growing), which can affect height as an adult

Talk with your child’s doctor if you have questions about potential side effects from Synthroid.

Note: Fontanelles are “soft spots” in an infant’s head where the skull bones haven’t fused together yet. Fontanelles usually finish closing at around 18 months of age.

Get answers to some frequently asked questions about Synthroid’s side effects.

What are the side effects of taking too much Synthroid?

Side effects of taking too much Synthroid are similar to symptoms of hyperthyroidism (overactive thyroid). These side effects can resemble Synthroid overdose symptoms and can include:

Do older adults have a higher risk of side effects from Synthroid?

Yes, people ages 65 years and older may have an increased risk of certain side effects from Synthroid than younger people.

For example, you may have a higher risk of certain heart conditions, such as atrial fibrillation, caused by Synthroid if you’re age 65 years or above. With atrial fibrillation, the upper parts of your heart don’t pump blood to the lower areas of your heart as well as they should. This could lead to complications, such as blood clots or stroke.

Due to this risk, if you’re an older adult, your doctor will likely start you on a lower-than-usual dosage of the drug. Talk with them if you have concerns about your risk of side effects from Synthroid due to your age.

Will I have side effects if my doctor lowers my Synthroid dosage?

It’s not likely. In Synthroid studies, most side effects reported happened when people had too much thyroid hormone in their body.

Your doctor will prescribe the dosage of Synthroid that provides the specific amount of thyroid hormone your body needs. If they lower your dosage to fit that need, you shouldn’t have side effects from the drug.

If your dose is too low, you may have symptoms of hypothyroidism. But these would be caused by your condition returning and not by Synthroid.

Symptoms of hypothyroidism can include:

It’s important to let your doctor know if you have symptoms of hypothyroidism while you’re taking Synthroid. They can adjust your dosage until your thyroid hormone is at the right level.

Does Synthroid cause any long-term side effects?

It’s possible. Although most common Synthroid side effects are mild and go away after a few days to weeks, it’s possible to have long-term side effects from this drug.

For example, thinning bones is a potential long-term side effect of Synthroid. If you experience thinning bones while taking Synthroid, this side effect will not resolve when you stop treatment. Like other side effects of the drug, this is more likely to happen if your dose is too high. Your risk is also higher if you’ve gone through menopause.

For more information about this side effect, see the “Tips for managing side effects” section.

If you have questions about the risk of long-term side effects with Synthroid, talk with your doctor.

Synthroid has a boxed warning regarding how it should not be used for weight loss or to treat obesity.. A boxed warning is the most serious warning from the Food and Drug Administration (FDA).

Boxed warning: Not to be used for weight loss or to treat obesity

Your thyroid helps control your body’s metabolism. Taking Synthroid can cause your thyroid hormones to reach levels that cause your body not to work normally. And this increases your risk of the drug reaching toxic levels in your body.

Using Synthroid for weight loss or to treat obesity can cause serious and potentially life threatening side effects that can lead to coma or death.

There may be things you can do to help make certain Synthroid side effects more manageable. If you have ongoing, worsening, or bothersome side effects, talk with your doctor or pharmacist. They can suggest tips for managing them or recommend other available treatment options for your condition.

Weight gain

It’s possible to experience weight gain while taking Synthroid. Increased appetite was a side effect reported in Synthroid studies. And increased appetite could lead to weight gain.

Weight gain may also be a symptom of hypothyroidism, which Synthroid is used to treat. So, it’s also possible to experience weight gain if your Synthroid dosage is too low. In this case, the weight gain would be due to your condition instead of the drug.

What might help

Talk with your doctor if you have weight changes while taking Synthroid. They can recommend ways to help you maintain a moderate weight. They can also do blood tests to check whether your Synthroid dosage is right for you. Then, they’ll adjust your dosage as needed.

Hair loss

You may have partial hair loss while taking Synthroid. This side effect is usually temporary and more common during your first few months of treatment.

What might help

Talk with your doctor if you have troublesome hair loss while taking Synthroid. They can suggest ways to help manage this side effect.

Insomnia

You may have insomnia while taking Synthroid. (With insomnia, you have trouble falling or staying asleep.) This side effect can happen if you have too much thyroid hormone in your body. If your Synthroid dosage is too high, your thyroid hormone levels may also be too high.

What might help

Let your doctor know if you have trouble sleeping while you’re taking Synthroid. They may check whether your thyroid hormone levels are too high. If needed, they can adjust your Synthroid dosage.

Thinning or weakened bones

Taking Synthroid may cause your bones to become thinner or weakened. The risk of this side effect is higher if your Synthroid dosage is too high. Your risk is also higher if you’ve gone through menopause.

What might help

To reduce your risk of thinning or weakened bones, your doctor will prescribe the lowest dosage of Synthroid that gives the desired effect. If you have a higher risk of weakened bones, your doctor may periodically check your bone strength using a bone density test.

Consider the following important information before starting Synthroid.

Synthroid has a boxed warning. A boxed warning is the most serious warning from the FDA. For details, see “Boxed warning for Synthroid.”

Other warnings

Synthroid may not be right for you if you have certain medical conditions. (This is known as a drug-condition interaction.) Other factors may also affect whether this drug is a good treatment option for you. Talk with your doctor about your health history before starting Synthroid. Factors to consider include those described below.

Heart problems. Synthroid may cause heart problems, such as fast or irregular heartbeat. If you already have a heart condition, such as atrial fibrillation, this drug could worsen your condition. Tell your doctor if you have any heart conditions before starting Synthroid. They’ll likely have you start the drug at a lower dosage than usual.

Allergic reaction. If you’ve had an allergic reaction to Synthroid or any of its ingredients, your doctor will likely not prescribe it for you. Ask them about other medications that might be better options.

Diabetes. Before starting Synthroid, let your doctor know if you have diabetes. This drug can make it difficult to manage your blood sugar. Because of this, your doctor will likely have you check your blood sugar level more frequently while you’re taking Synthroid.

Myxedema coma. Synthroid should not be used to treat myxedema coma. This is a life threatening condition that requires emergency medical attention. It’s very important that someone seek emergency medical treatment on your behalf if you experience myxedema coma.

Adrenal gland problems. Before starting Synthroid treatment, let your doctor know if you have adrenal gland problems, such as adrenal insufficiency. With this condition, your body doesn’t make enough adrenal hormones, including cortisol. Taking Synthroid can lower your adrenal hormone levels even more. Before you start Synthroid, your doctor may give you treatments to increase your adrenal hormone levels.

Alcohol and Synthroid

There aren’t any known interactions between alcohol and Synthroid. If you drink alcohol, talk with your doctor about an amount that’s safe to consume while you take this drug.

Pregnancy and breastfeeding while taking Synthroid

Synthroid is considered safe to take during pregnancy.

Having hypothyroidism (which Synthroid is used to treat) during pregnancy may increase certain risks, including pregnancy loss or early delivery. Due to these risks, your doctor may recommend that you take Synthroid while you’re pregnant. In some cases, they may need to adjust your dosage during this time.

Synthroid passes into breast milk, but it’s not known whether the drug is safe for a breastfed baby.

Talk with your doctor if you’re pregnant, planning to become pregnant, breastfeeding, or planning to breastfeed while taking Synthroid. They can discuss with you the potential risks and benefits of taking this drug during these times.

Disclaimer: Healthline 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.