Key Takeaways

  • Common side effects of levothyroxine include tremors, insomnia, heart-related problems, headache, and muscle spasms or muscle weakness.
  • It’s possible for levothyroxine to cause long-term side effects, but most of its side effects lessen over time.

Levothyroxine is used in adults and certain children to treat hypothyroidism. It’s also used in adults to treat thyroid cancer in certain situations.

Keep reading to learn about the common, mild, and serious side effects levothyroxine can cause. To learn more about levothyroxine, including its uses, dosage, and cost, see this levothyroxine overview.

Levothyroxine 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 levothyroxine” section.

Note: Most side effects that people taking levothyroxine experience are due to taking too high of a dose. Once your doctor determines the right dosage for you, you’ll likely have few to no side effects.

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Levothyroxine can cause certain side effects, some of which are more common than others. These side effects may be temporary, lasting a few days to weeks. Talk with your doctor if side effects do not go away, get worse, or become too difficult to tolerate.

Examples of levothyroxine’s commonly reported side effects include:

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

Mild side effects have been reported with levothyroxine. These include:

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

Levothyroxine may cause mild side effects other than the ones 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.”

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

If you develop serious side effects while taking levothyroxine, 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 levothyroxine and want to tell the FDA about it, visit MedWatch or call 800-FDA-1088.

Allergic reaction

For some people, levothyroxine can cause an allergic reaction.

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 levothyroxine 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 levothyroxine, they may have you switch to a different treatment.

In most cases, children who take levothyroxine can have side effects similar to those adults reported from the drug. But children may have other side effects with levothyroxine.

For example, levothyroxine can cause pseudotumor cerebri in children. With this condition, pressure builds up in the brain. Symptoms include headache, blurry vision, nausea, and vomiting.

In addition, a hip condition called slipped capital femoral epiphysis (SCFE) can occur in children taking levothyroxine. With SCFE, the top of the thigh bone, referred to as the ball of the hip joint, starts to separate from the rest of the bone. This condition can affect the hip joint if a child is still growing. A symptom of SCFE is joint pain.

In infants, levothyroxine can cause craniosynostosis. This condition occurs when the skull bones fuse too early, which can result in increased pressure in the brain. It’s also possible that when a child takes levothyroxine, their growth plates close too early. (Growth plates are areas of new bone growth at the end of certain bones in children.) This can affect the child’s growth.

These levothyroxine side effects specific to children are not common. If your child takes this drug, their doctor can monitor them for symptoms of any side effects during treatment. Discuss with them any concerns or questions you may have about levothyroxine side effects in children.

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

Does levothyroxine cause long-term side effects?

Yes, it’s possible for levothyroxine to cause long-term side effects, though most of its side effects lessen over time. Examples of long-term side effects that levothyroxine may cause include:

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

Are hair loss and weight gain side effects of levothyroxine?

Levothyroxine can cause hair loss to occur. This side effect was reported in studies of levothyroxine, but it was not common.

If you have hair loss from levothyroxine that bothers you, talk with your doctor. They may be able to recommend ways to decrease this side effect. For example, an over-the-counter medication such as minoxidil (Rogaine) may help manage hair loss.

Weight gain was not a reported side effect of levothyroxine. So you shouldn’t have weight gain from taking levothyroxine. In fact, studies reported weight loss as a side effect of levothyroxine.

If you notice weight gain during your treatment with levothyroxine, let your doctor know. Other side effects of the drug, such as heart failure, may cause weight gain. Your doctor will determine what could be causing the weight gain and recommend possible treatments.

Also, weight gain may mean that your body doesn’t have enough thyroid hormone. In this case, your doctor may adjust your dose of levothyroxine.

Can stopping levothyroxine treatment abruptly result in withdrawal symptoms?

No, you shouldn’t have withdrawal symptoms from stopping levothyroxine abruptly. Withdrawal symptoms can occur if you stop taking a medication that your body has become used to.

But stopping levothyroxine treatment may cause your thyroid condition to worsen. If you need to stop taking levothyroxine, talk with your doctor.

Does levothyroxine cause eye-related side effects?

No, you shouldn’t have eye-related side effects from taking levothyroxine. Eye-related side effects weren’t reported in studies of people taking this medication.

However, it’s possible for hyperthyroidism (high levels of thyroid hormone) to cause thyroid eye disease. Symptoms of thyroid eye disease include:

If you develop eye problems during your treatment with levothyroxine, talk with your doctor. They’ll try to determine what is causing your eye problems and the best ways to treat them.

Levothyroxine has a boxed warning that 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).

Levothyroxine may cause weight loss to occur. But if your thyroid is working normally, taking levothyroxine is not an effective option for weight loss. In addition, taking high doses of the drug may cause serious side effects, such as:

Sometimes, these side effects can be life threatening or even fatal.

If you have concerns about weight management, talk with your doctor. They can suggest safe ways to lose or manage weight.

There may be things you can do to help make certain levothyroxine 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.

Heart-related problems, such as heart palpitations

Levothyroxine may cause heart-related problems to occur.

Heart-related problems that were reported in studies of levothyroxine include:

It’s not known how often heart-related side effects occurred in these studies.

Depending on the heart-related problem, symptoms vary but may include:

  • chest pain
  • dizziness
  • shortness of breath
  • increased blood pressure
  • edema (swelling from fluid buildup)

What might help

If you have any symptoms of heart-related side effects from taking levothyroxine, tell your doctor right away. They’ll likely have you get an electrocardiogram (EKG) to look at your heart’s electrical activity as well as check your thyroid hormone levels. They may lower your dosage of levothyroxine if your thyroid hormone levels are too high.

Weight loss

It’s possible to have weight loss from taking levothyroxine. While this side effect was reported in studies of this medication, it’s unclear how often it occurred.

Note that levothyroxine has a boxed warning that 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). It alerts doctors and patients about drug effects that may be dangerous.

If your thyroid is working as it should, taking levothyroxine is not an effective option for weight loss. And taking too much of this drug to try to lose weight can result in serious side effects. For more information about this boxed warning, see the “Boxed warning for levothyroxine” section.

What might help

If you have weight loss while taking levothyroxine, talk with your doctor. In some cases, they may check your thyroid hormone levels through blood tests to make sure that your dose of levothyroxine is not too high. They may also recommend nutrition plans to help manage your weight or refer you to a dietitian.

Diarrhea

Levothyroxine can cause diarrhea. Studies didn’t indicate how often this side effect occurred in people taking the drug.

What might help

If you develop diarrhea during your treatment with levothyroxine, talk with your doctor. They may be able to suggest ways to lessen this side effect. For example, they may recommend an over-the-counter medication to treat diarrhea that’s severe or bothersome to you. An example is Imodium (loperamide).

For diarrhea remedies to try at home, see this article. Your doctor may also be able to suggest other at-home treatments for diarrhea.

Headache

You may have a headache from taking levothyroxine. Studies of levothyroxine reported this side effect but didn’t indicate how often it occurred.

What might help

If you have headaches from taking levothyroxine, talk with your doctor. They may be able to recommend ways to manage your headaches. For example, an over-the-counter pain medication, such as acetaminophen (Tylenol) or ibuprofen (Advil), may help to manage your headaches.

For tips on easing headaches, see this article.

Muscle spasms or muscle weakness

It’s possible to have muscle spasms or muscle weakness from taking levothyroxine. How frequently muscle spasms or muscle weakness occurred in studies wasn’t reported.

Symptoms of these side effects include:

What might help

If you develop symptoms of muscle spasms or muscle weakness from taking levothyroxine, talk with your doctor. They can discuss ways to manage these side effects. They may also check your thyroid hormone levels. If these are high, your doctor will likely lower your dosage of levothyroxine.

Consider the following important information before starting levothyroxine.

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

Other precautions

Before taking levothyroxine, discuss your health history with your doctor. Levothyroxine may not be right for you if you have certain medical conditions or other factors affecting your health. Be sure to talk with your doctor if any of the following apply to you:

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.