If you have a thyroid condition, you may be interested in learning more about levothyroxine. It’s a generic prescription drug used to treat:

For these purposes, the drug is prescribed to adults and some children.

Levothyroxine comes as oral tablets (which are tablets that you swallow). It also comes in injectable forms and other oral forms, such as capsules and a liquid solution. Only levothyroxine oral tablets are covered in this article.

If levothyroxine works for you, your doctor will likely recommend that you take it long term.

This article describes levothyroxine’s side effects, also referred to as adverse effects. For more information about levothyroxine, including details about its uses, see this in-depth article.

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.

Some people may experience mild to serious side effects during their levothyroxine treatment. Examples of levothyroxine’s commonly reported side effects include:

* To learn more about this side effect, see the “Side effects explained” section below.

Mild side effects may occur from taking levothyroxine. Examples of mild side effects that have been reported with levothyroxine include:

* To learn more about this side effect, see the “Side effects explained” section below.

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.

Note: After the Food and Drug Administration (FDA) approves a drug, it tracks side effects of the medication. If you’d like to notify the FDA about a side effect you’ve had with levothyroxine, visit MedWatch.

It’s possible to develop serious side effects from taking levothyroxine, though they aren’t common. Serious side effects that have been reported with levothyroxine include:

* To learn more about this side effect, see the “Side effects explained” section below.

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.

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.

* To learn more about this side effect, see the “Side effects explained” section below.

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.

Learn more about some of the side effects levothyroxine may cause.

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 “Levothyroxine warnings” section below.

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.

Allergic reaction

Like most drugs, levothyroxine can cause an allergic reaction in some people.

Symptoms can be mild to serious and can include:

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

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. Examples include:

  • an antihistamine you take by mouth, such as Benadryl (diphenhydramine)
  • a product you apply to your skin, such as hydrocortisone cream

If your doctor confirms you’ve had a mild allergic reaction to levothyroxine, they’ll decide if you should continue using it.

If you have symptoms of a severe allergic reaction, such as swelling or trouble 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.

Keeping track of side effects

During your levothyroxine treatment, consider taking notes on any side effects you’re having. You can then share this information with your doctor. This is especially helpful when you start taking new drugs or using a combination of treatments.

Your side effect notes can include things such as:

  • what dose of the drug you were taking when you had the side effect
  • how soon you had the side effect after starting that dose
  • what your symptoms were
  • how it affected your daily activities
  • what other medications you were taking
  • any other information you feel is important

Keeping notes and sharing them with your doctor will help them learn more about how levothyroxine affects you. They can then use this information to adjust your treatment plan if needed.

Levothyroxine comes with several warnings, which may affect whether the drug is a good treatment for your condition.

Boxed warning: Not for weight loss or treatment of obesity

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.

Other warnings

Levothyroxine may not be right for you if you have certain medical conditions. These are known as drug-condition interactions. Other factors may also affect whether levothyroxine is a good treatment option for you.

Talk with your doctor about your health history before starting levothyroxine. Factors to consider include those described below.

Heart problems. Levothyroxine can cause heart-related side effects, such as an increased heart rate or heart palpitations. As a result, it can worsen heart problems in people who have them. If you have a heart problem, tell your doctor before you start taking levothyroxine. In this case, your doctor will likely monitor your heart throughout levothyroxine treatment. They may also prescribe a lower dose of the drug.

Adrenal gland problems. In some cases, levothyroxine may affect adrenal gland hormones. If you have an adrenal gland problem, such as Addison’s disease, taking levothyroxine may worsen your condition. Before you start taking levothyroxine, tell your doctor about any adrenal gland problems you might have. Your doctor may recommend treating this condition before starting levothyroxine treatment.

Osteoporosis. Levothyroxine can weaken bones, so taking it could worsen osteoporosis. If you have osteoporosis, tell your doctor before you start taking this drug. Your doctor may recommend monitoring your bone density more often during levothyroxine treatment.

Blood clotting problems. Levothyroxine can increase the effects of certain blood thinners. This in turn may increase your risk of bleeding, especially if you have a blood clotting problem. Before you start levothyroxine treatment, tell your doctor about any blood clotting problems that you have. They may monitor your blood more often throughout treatment with levothyroxine. They may also adjust your dosage of the blood thinner you take.

Allergic reaction. If you’ve had an allergic reaction to levothyroxine or any of its ingredients, your doctor will likely not prescribe levothyroxine. Ask them what other medications are better options for you.

Myxedema crisis. It’s not recommended to take levothyroxine to treat myxedema crisis, also called myxedema coma. This condition can occur when thyroid hormone levels become very low. In most cases, myxedema crisis requires treatment in a hospital setting. If you develop this condition, your doctor will determine the best way to treat it.

Diabetes. Levothyroxine may increase your blood sugar levels. If you have diabetes, this drug may worsen your condition. Before starting levothyroxine treatment, let your doctor know if you have diabetes. They may recommend monitoring your blood sugar levels more often while taking this drug.

Alcohol and levothyroxine

There aren’t any known interactions between alcohol and levothyroxine.

If you have any questions about drinking alcohol while taking levothyroxine, talk with your doctor.

Pregnancy and breastfeeding while taking levothyroxine

It’s safe to take levothyroxine during pregnancy. But it’s not known if the drug is safe to take while breastfeeding.

Studies of levothyroxine during pregnancy have not shown an increased risk of fetal harm or pregnancy loss. But untreated thyroid conditions may cause harm to someone who is pregnant or a developing fetus. So it’s important to treat thyroid conditions during pregnancy.

If you take levothyroxine during pregnancy, your doctor will monitor your thyroid hormones through blood tests. You may need a higher dose of levothyroxine while pregnant. Your doctor can recommend the appropriate dose for you during this time.

Levothyroxine can pass into breast milk. But it’s unknown what effects it may have on a child who’s breastfed by someone taking the drug.

If you’re pregnant or breastfeeding, or planning to become pregnant or breastfeed, talk with your doctor before taking levothyroxine. They can discuss with you the best treatment plan for your thyroid condition during this period.

You may experience side effects from taking levothyroxine, but usually, these are mild. 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 experience few to no side effects.

If you have questions or concerns about side effects from this drug, talk with your doctor or pharmacist. Here are a few questions you may wish to ask:

  • Do older people have a higher risk of side effects from levothyroxine?
  • Am I at an increased risk of levothyroxine side effects because of other medications that I take?
  • Do my other medical conditions increase my risk of side effects from this drug?
  • Is the risk of side effects higher when starting levothyroxine treatment?

If you have any questions about side effects that levothyroxine can cause, talk with your doctor. You can also ask them about Euthyrox, Levoxyl, Synthroid, and Unithroid, which are brand-name versions of levothyroxine. A generic drug and its brand-name version are expected to have the same side effects because they contain the same active ingredient. (An active ingredient is what makes a drug work.)

For more about levothyroxine or Synthroid, see these articles:

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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.