Unithroid (levothyroxine) is a brand-name oral tablet that’s prescribed for certain thyroid conditions in adults and some children. As with other drugs, Unithroid can cause side effects, such as nervousness and heat intolerance.

Unithroid 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. However, if the side effects last longer than that, bother you, or become severe, be sure to talk with your doctor or pharmacist.

These are just a few of the more common side effects reported by people who took Unithroid in clinical trials:

  • headache
  • anxiety, irritability, or nervousness
  • insomnia
  • diarrhea
  • muscle tremors, spasms, or weakness

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

Mild side effects that have been reported with Unithroid include:

These side effects may be temporary, lasting a few days to weeks. However, 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 Unithroid and want to tell the FDA about it, visit MedWatch.

* For more information about this side effect, see “Side effect specifics” below.
† An allergic reaction is possible after taking Unithroid. However, this side effect wasn’t reported in clinical trials. To learn more, see the “Side effect specifics” section below.

Unithroid may cause serious side effects, although they aren’t common. The list below may not include all possible serious side effects of the drug. For more information, you can refer to Unithroid’s prescribing information.

If you develop serious side effects while taking Unithroid, 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 include:

* For more information about this side effect, see “Side effect specifics” below.
† An allergic reaction is possible after taking Unithroid. However, this side effect wasn’t reported in clinical trials. To learn more, see the “Side effect specifics” section below.

The side effects of Unithroid seen in children and adults are similar.

However, some additional side effects were only reported in infants and children in clinical trials of levothyroxine. (Levothyroxine is the active drug in Unithroid.)

Additional side effects may include:

If you have questions or concerns about how Unithroid may affect your child, talk with their doctor.

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

How do side effects of Unithroid compare with those seen with Synthroid?

Unithroid and Synthroid both contain the same active drug, levothyroxine. Because of this, they’re both expected to work the same way and cause the same side effects.

However, these drugs are not identical. Unithroid and Synthroid contain certain different inactive ingredients. These ingredients may cause reactions in people with allergies or sensitivities to them. For this reason, it’s possible to experience an allergic reaction with one drug and not the other.

Before you start treatment with Unithroid or Synthroid, talk with your doctor about any allergies you may have. They’ll determine the safest treatment option for you.

Is Unithroid safe for older adults?

Yes, Unithroid is safe for older adults to take. However, older adults may have a higher risk of heart problems, such as an irregular heartbeat or fast heart rate. If you’re age 65 years or older, your doctor will likely start you on a lower dose of Unithroid.

Talk with your doctor if you have any questions about possible side effects while taking the drug.

Learn more about some of the side effects that Unithroid may cause. To find out how often side effects occurred in clinical trials, see the prescribing information for Unithroid.

Heart problems

It’s possible for Unithroid to cause heart problems, such as an irregular heartbeat or a heart attack. However, these weren’t commonly reported side effects in the drug’s clinical trials.

The risk of heart problems with Unithroid is typically associated with excess thyroid hormone. If you take more Unithroid than necessary, you may have a higher risk of this side effect. The risk may also be higher in older adults and people with existing heart conditions.

Symptoms of a heart problem can include:

What you can do

Tell your doctor if you experience a fast or irregular heartbeat. Your doctor will likely do a blood test to check your thyroid hormone levels. If your levels are too high, your doctor may lower your dose or have you temporarily stop taking the medication.

However, if you experience chest pain or difficulty breathing, seek immediate medical care.

Heat intolerance

Unithroid may cause heat intolerance. This was a more common side effect in the drug’s clinical trials. With heat intolerance, you may feel unusually warm or hot in temperatures that others find comfortable.

Thyroid hormones play an important role in regulating body temperature. Taking Unithroid increases the level of thyroid hormones in your body. In turn, this can affect your body temperature and make you more sensitive to heat. Symptoms of heat intolerance may include:

  • excessive sweating
  • headache
  • nausea
  • fever
  • flushing in your face or chest (warmth and temporary redness or deepening of skin color)

What you can do

There are a few things you can do to help manage heat intolerance. These include:

  • drinking plenty of fluids to stay hydrated
  • limiting your time outdoors in hot or humid weather
  • wearing loose-fitting or breathable clothing

If you experience heat intolerance that feels bothersome or severe, talk with your doctor. They can test your thyroid hormone levels. It’s possible that adjusting your dose of Unithroid may help resolve this side effect.

If you experience heat intolerance that causes a fever or confusion, seek medical attention immediately.

Allergic reaction

As with most drugs, Unithroid can cause an allergic reaction in some people. This side effect wasn’t reported in the drug’s clinical trials. However, there have been rare reports of allergic reactions to Unithroid’s inactive ingredients since the drug was approved for use.

Symptoms can be mild or serious and can include:

  • skin rash
  • itching
  • flushing
  • swelling under your skin, typically in your eyelids, lips, hands, or feet
  • swelling of your mouth, tongue, or throat, which can make it hard to breathe

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 Unithroid. However, if your symptoms are serious and you think you’re having a medical emergency, immediately call 911 or your local emergency number.

Below is important information you should consider before taking Unithroid.

Boxed warning: Not for the treatment of obesity or weight loss

This drug has a boxed warning stating that it is not for the treatment of obesity or weight loss. A boxed warning is the most serious warning from the FDA.

Unithroid is not approved to treat obesity or to help with weight loss. In people with typical thyroid hormone levels, the drug is not effective for weight loss. However, weight loss is a side effect of hyperthyroidism. If you take more Unithroid than your body needs, it may cause hyperthyroidism.

Taking Unithroid for weight loss may lead to harmful side effects. Some side effects, such as confusion, may be mild. However, other side effects can become life threatening, such as a seizure or serious blood clot.

If you’re interested in losing weight, talk with your doctor. They can discuss safe weight management options for you.

Other precautions

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

Heart disease: If you have heart disease, you may have a higher risk of developing a heart problem with Unithroid. This can include an increased heart rate, irregular heartbeat, or heart attack. Before you start taking this drug, tell your doctor if you have a heart problem. They may prescribe a lower dose of Unithroid than they typically would.

Diabetes: If you have diabetes, taking Unithroid could increase your blood sugar. At the start of treatment and after a dosage change, it’s important to closely monitor your blood sugar. Your doctor may need to adjust your dosage of Unithroid or insulin to manage your blood sugar.

Adrenal insufficiency: If you have adrenal insufficiency (low cortisol levels), talk with your doctor. It’s possible that taking Unithroid may worsen your condition. Due to this risk, your doctor will likely want to treat your condition before prescribing Unithroid.

Myxedema coma: A myxedema coma is a complication of severe hypothyroidism (also called myxedema). This is a medical emergency resulting from dangerously low thyroid hormone levels. It can cause seizures, respiratory depression, and lethargy. Unithroid is not approved to treat a myxedema coma. If you experience symptoms of this condition, call 911 or your local emergency number.

Osteoporosis: Unithroid can cause a loss of bone density, which can lead to osteoporosis. The risk of this side effect may be higher in females* after menopause. Before you start taking Unithroid, talk with your doctor about your potential risk factors for osteoporosis. They will determine whether this drug is safe for you to take.

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

Pregnancy or breastfeeding: If you’re pregnant or breastfeeding your child, Unithroid is likely safe for you to take. However, it’s important to talk with your doctor about the possible effects of Unithroid during pregnancy and while breastfeeding. They will help you weigh the benefits and the possible risks of treatment during this time.

Alcohol consumption: There is no known interaction between Unithroid and alcohol. Your doctor or pharmacist can advise how much alcohol, if any, is safe to drink during treatment.

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

Like most medications, treatment with Unithroid can cause side effects. Most side effects are mild if they do occur. Talk with your doctor if you have any side effects that last, are bothersome, or are serious.

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

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

  • More information about Unithroid: For details about other aspects of Unithroid, refer to this article.
  • Cost: If you’d like to learn about Unithroid and cost, see this article.
  • Dosage: For information about the dosage of Unithroid, view this article.
  • Interactions: To find out about Synthroid’s interactions, see this article. (Synthroid contains the same active drug as Unithroid.)
  • A look at your thyroid health: For details about hypothyroidism, see our list of hypothyroid articles.

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.