Unithroid (levothyroxine) is a prescription drug that’s used to treat specific thyroid diseases. Unithroid can cause side effects that range from mild to serious. Examples include shortness of breath, irregular heart rate, and heart attack.

Specifically, Unithroid is used in adults and children to:

Unithroid comes as an oral tablet and contains the active ingredient levothyroxine. (An active ingredient is what makes a drug work.)

Keep reading to learn about the common, mild, and serious side effects Unithroid can cause. For a general overview of the drug, including details about its uses, see this article.

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

These are not all of the side effects the drug may cause, and not everyone will experience them. Keep reading to learn about other mild to serious side effects Unithroid may cause.

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

In most cases, these side effects should be temporary. And your doctor can help manage some side effects by adjusting your dose. But if you have symptoms that are ongoing or bothersome, talk with your doctor or pharmacist. And do not stop taking Unithroid unless your doctor recommends it.

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

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

Serious side effects have been reported with Unithroid. These include:

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.

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

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

Unithroid tablets are prescribed for infants and children of all ages to:

The following side effects were more commonly reported in children than in adults in the drug’s studies.

  • increased pressure inside skull
  • hip joint problems
  • premature skull bone fusion in infants
  • stopped growth of certain leg and arm bones in children
  • shortened height as an adult

For more information about potential side effects of Unithroid in children, talk with your doctor or pharmacist.

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

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

Unithroid and Synthroid are both brand-name versions of the generic drug levothyroxine. They work in the same way to provide thyroid hormone if your body isn’t making enough. They have the same side effects overall.

However, even though the active ingredient* is the same, there may be some variation in the other inactive filler ingredients. Inactive fillers can cause side effects by affecting how the pill is absorbed or affecting individuals with allergies or sensitivities.

Talk with your pharmacist or doctor if you have questions about inactive ingredients, allergies, or sensitivities.

* The active ingredient (levothyroxine) is what makes the drug work.

Does Unithroid cause long-term side effects?

Yes, it can. Long-term side effects include those that may start at any time you’re taking a drug, even if you’ve taken it for a long time. It also includes side effects that may not go away even after you stop taking the drug.

Examples of long-term side effects reported in Unithroid’s studies include:

  • weakened bones
  • heart damage
  • shortened height as an adult

Talk with your doctor to learn more about possible side effects of Unithroid and how long they may last.

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

Heat intolerance

Heat intolerance was a common side effect reported in studies of Unithroid. By stimulating your thyroid, Unithroid can boost your body’s energy production. This is like turning up the heat in your body, which can make you feel too hot, sweaty, or like you can’t manage heat the same way you usually would.

Heat intolerance may cause symptoms such as:

  • flushing (temporary warmth, redness, or deepening of skin color) in the face
  • skin that’s hot to the touch
  • headache
  • muscle weakness or cramping
  • nausea
  • excessive sweating
  • dizziness
  • fever

Factors that can increase the risk of heat intolerance when taking this drug include:

  • dehydration (low fluid level)
  • too high a dose of Unithroid
  • hot weather (heat advisories)
  • tight-fitting clothes, such as those made with polyester

What might help

If you have heat intolerance during Unithroid treatment, consider the following options to help relieve your symptoms:

  • remain indoors with a fan or air conditioning on
  • drink cool water
  • take a lukewarm shower
  • wear loose, cotton clothing

If heat intolerance causes fever, confusion, lethargy, shortness of breath, or a pounding heart, seek medical attention immediately.

Weakened bones

Osteoporosis (bone weakness) was a common side effect reported in studies of Unithroid. Weak bones may cause symptoms such as:

  • severe back pain
  • hunched back
  • broken bones
  • decreased height

Factors that can increase the risk of bone weakness while taking this drug include:

  • taking too high a dose of Unithroid
  • being in menopause or postmenopause
  • physical inactivity
  • being age 65 years or older
  • having a family history of osteoporosis
  • being female*
  • having a small body frame
  • being Caucasian or Asian

* In this article, we use the term “female” to refer to someone’s sex assigned at birth. For information about the difference between sex and gender, see this article.

What might help

If you develop symptoms of osteoporosis during Unithroid treatment, talk with your doctor. They’ll likely recommend testing your bone density. If your bones are weak, they’ll likely suggest medical treatment, dietary changes, and increased physical activity.

Allergic reaction

Like most drugs, Unithroid 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 oral antihistamine, 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 Unithroid, they’ll decide whether you should continue taking it.

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

Keeping track of side effects

During your Unithroid 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 first start taking a new drug 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 your symptoms 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 Unithroid affects you. They can then use this information to adjust your treatment plan if needed.

Below is important information you should consider before starting Unithroid.

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

Unithroid has a boxed warning about not being used for weight loss or the treatment of obesity. A boxed warning is the most serious warning from the Food and Drug Administration (FDA).

You should not take Unithroid by itself or with other weight loss drugs to lose weight. If your thyroid is working as it should and you take these hormones to try to lose weight, it won’t help. And if you take too much, the results could be severe, including heart attack, heart failure, seizure, stroke, coma, or even death.

If you need medication for weight loss, speak with your pharmacist or doctor. They can suggest safer options.

Other warnings

Unithroid can sometimes cause harmful effects in people with certain 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 Unithroid. Be sure to tell them if any of the following factors apply to you:

Alcohol and Unithroid

There are no known interactions between alcohol and Unithroid. But Unithroid should be taken with water on an empty stomach. Other food or drinks can decrease the absorption of this drug and reduce its effectiveness.

If you have questions about consuming alcohol while taking Unithroid, talk with your doctor or pharmacist.

Pregnancy and breastfeeding with Unithroid

Before starting Unithroid treatment, it’s important to talk with your doctor if you’re pregnant, breastfeeding, or planning to be either.

Pregnancy

Unithroid is generally considered safe to take during pregnancy. Studies have shown that taking Unithroid during pregnancy hasn’t led to an increase in pregnancy loss or problems with fetal development (commonly known as birth defects).

More importantly, not treating an underactive thyroid during pregnancy can harm both mother and fetus.

If you’re pregnant or planning a pregnancy, you should still talk with your doctor before starting treatment with Unithroid.

Breastfeeding

Unithroid is generally considered safe to take while breastfeeding. Some research shows levothyroxine can pass into breast milk, but it’s not clear how it affects milk supply or a child who is breastfed. Treatment for thyroid problems may help people who are breastfeeding to make enough milk.

If you’re breastfeeding or planning to do so, talk with your doctor before starting Unithroid treatment.

Like most drugs, Unithroid can cause a number of side effects that range from mild to serious. But most are temporary and go away after a few days to weeks. If you have questions about side effects this drug can cause, talk with your doctor. Examples to help get you started include:

  • Can any of Unithroid’s side effects affect my daily activities, such as driving or working?
  • Will side effects likely go away over time as my body adjusts to the medication?
  • Which side effects of Unithroid should I contact you about right away?

To learn more about Unithroid, 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.