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Generic name: levothyroxine (oral/injection) [ LEE-voe-thye-ROX-een ]
Brand names: Ermeza, Euthyrox, Levo-T, Levoxyl, Synthroid, ... show all 9 brands
Drug class: Thyroid drugs

Medically reviewed by Carmen Pope, BPharm. Last updated on Jan 4, 2024.

What is levothyroxine?

Levothyroxine is used in adults and children to treat hypothyroidism (underactive thyroid - a condition where the thyroid gland does not produce enough thyroid hormone).

Levothyroxine is also used in adults along with surgery and radioactive iodine therapy to treat a certain type of thyroid cancer.

Levothyroxine injection is used in adults to treat myxedema coma.

Levothyroxine belongs to a class of medications called hormones. It works by replacing thyroid hormone that is normally produced by the body.

Without thyroid hormone, your body cannot function properly, which may result in poor growth, slow speech, lack of energy, excessive tiredness, constipation, weight gain, hair loss, dry, thick skin, increased sensitivity to cold, joint and muscle pain, heavy or irregular menstrual periods, and depression. When taken correctly, levothyroxine reverses these symptoms.

Levothyroxine side effects

Get emergency medical help if you have signs of an allergic reaction to levothyroxine: hives, difficult breathing, swelling of your face, lips, tongue, or throat.

Levothyroxine may cause serious side effects. Call your doctor at once if you have:

Common levothyroxine side effects may include:

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.


You may not be able to use levothyroxine if you have certain medical conditions. Tell your doctor if you have an untreated or uncontrolled adrenal gland disorder or any heart problems such as a recent heart attack.

Levothyroxine should not be used to treat obesity or weight problems. Dangerous side effects or death can occur from the misuse of this medicine, especially if you are taking any other weight-loss medications or appetite suppressants.

Taking more than your recommended dose will not make this medicine more effective, and may cause serious side effects.

Before taking this medicine

You should not use levothyroxine if you are allergic to glycerin or edetate disodium, or if you have an untreated or uncontrolled adrenal gland disorder.

To make sure this medicine is safe for you, tell your doctor if you have ever had:

Tell your doctor if you are pregnant or plan to become pregnant. Having hypothyroidism during pregnancy may increase the risk of premature birth or other complications. The benefit of treating hypothyroidism may outweigh any risks to the baby. Your dose needs may be different during pregnancy.

Tell your doctor if you are breastfeeding. Your dose needs may be different while you are nursing.

You may be more likely to have a broken bone while using levothyroxine. Talk with your doctor about ways to keep your bones healthy.

How should I use levothyroxine?

Take levothyroxine exactly as prescribed by your doctor. Follow all directions on your prescription label and read all medication guides or instruction sheets. Your doctor may occasionally change your dose.

Some brands of levothyroxine have different dosage instructions and/or recommendations of how soon you need to take them before eating food. Always check the instructions on the label. 

Oral levothyroxine

Oral levothyroxine is taken by mouth and is available as tablets, capsules, and an oral solution.

Tablets and capsules

Take levothyroxine tablets and capsules on an empty stomach, at least 30 to 60 minutes before breakfast with a full glass of water. Take the medicine at the same time each day.

Oral solution (liquid medicine)

Measure liquid medicine with the supplied measuring device (not a kitchen spoon).

Intravenous levothyroxine

This is given as an injection into a vein by a healthcare provider and should only be used to treat myxedema coma.

What happens if I miss a dose?

In a medical setting, you are not likely to miss a dose of levothyroxine injection.

Take the missed oral levothyroxine dose as soon as you remember, and then go back to your regular schedule. Do not use two doses at one time.

What happens if I overdose?

Seek emergency medical attention or call the Poison Help line at 1-800-222-1222. An overdose can be fatal.

Overdose symptoms may include headache, leg cramps, tremors, feeling nervous or irritable, chest pain, shortness of breath, fast or pounding heartbeats, stroke, and coma.

What should I avoid while using levothyroxine?

Avoid the following food products within 1 hour of taking oral levothyroxine or the medication will not be as effective: grapefruit juice, infant soy formula, soybean flour, cotton seed meal, walnuts, and high-fiber foods.

What other drugs will affect levothyroxine?

Sometimes it is not safe to use certain medicines at the same time. Some drugs can affect your thyroid hormone levels and also make levothyroxine less effective.

If you use any of the following drugs, avoid taking them within 4 hours before or 4 hours after you use levothyroxine:

Tell your doctor about all your other medicines, especially:

This list is not complete and many other drugs may interact with levothyroxine. This includes prescription and over-the-counter medicines, vitamins, and herbal products. Not all possible drug interactions are listed here.

Popular FAQ

In most cases, levothyroxine causes some weight loss. According to the American Thyroid Association, when this medication is started, you may lose up to 10% of your weight. This weight is mainly water weight, since being hypothyroid makes you retain water. Continue reading

Most foods are considered fine to eat for breakfast as long as they are eaten 30 to 60 minutes after taking levothyroxine. Levothyroxine should be taken once a day on an empty stomach in the morning. Continue reading

Levothyroxine is known to interact with many other medications (called a drug-drug interaction), but there are different kinds of drug-drug interactions. Some interactions require that drugs not be taken at the same time, while other interactions require more frequent laboratory testing or monitoring of your disease symptoms. Continue reading

Yes, levothyroxine, the main treatment for a sluggish thyroid gland, may cause hair loss in addition to other side effects. You may experience partial hair loss in the first few months of treatment, but this often resolves and goes back to normal in a short time. Continue reading

You should avoid drinking milk until at least 4 hours after taking levothyroxine. Dairy products—including milk, cheese and yogurt—contain high calcium levels, which affect how levothyroxine is absorbed into your body. Continue reading

Levothyroxine will stay in your system for around 4 to 6 weeks. It takes longer for people with hypothyroidism to eliminate levothyroxine (closer to 6 weeks) than those with normal thyroid function. Continue reading

Levoxyl and Synthroid are brand names for the drug levothyroxine. Both medications contain the same active ingredient, levothyroxine, but they contain different inactive ingredients. Continue reading

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Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use levothyroxine only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.