Pronunciation: LEE voe thye ROX een
Generic name: levothyroxine
Brand names: Ermeza, Euthyrox, Levo-T, Levoxyl, Synthroid, ... show all 9 brands Thyquidity, Tirosint, Tirosint-Sol, Unithroid
Dosage form: oral tablets, oral capsules, oral solution, injection
Drug class: Thyroid drugs

Medically reviewed by Carmen Pope, Senior Medical Editor, B. Pharm. Last updated on Aug 28, 2026.

What is levothyroxine?

Levothyroxine is a man-made form of thyroxine (T4), one of the hormones your thyroid gland normally produces. It belongs to a class of medications called hormones, and it works by replacing thyroid hormone that your body is not making enough of.

Levothyroxine is used in adults and children, including newborn babies, to:

• treat hypothyroidism (underactive thyroid - a condition where the thyroid gland does not produce enough thyroid hormone). It replaces the missing hormone whether the condition was present from birth or developed later, and whether the problem starts in the thyroid gland itself or in the pituitary gland or hypothalamus; and
• lower the level of thyroid-stimulating hormone (TSH) in the blood in people being treated for a certain type of thyroid cancer, alongside surgery and radioactive iodine therapy.

Levothyroxine injection is used in adults to treat myxedema coma, a rare and life-threatening complication of a severely underactive thyroid.

Without thyroid hormone, your body cannot function properly. This 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, although it may take several weeks before you notice a difference.

Levothyroxine is usually taken for life.

What levothyroxine is not used for

Levothyroxine is not used to shrink benign (noncancerous) thyroid nodules or a nontoxic diffuse goiter in people who get enough iodine in their diet. It does not help in these situations, and taking more thyroid hormone than your body needs can cause an overactive thyroid. It is also not used to treat hypothyroidism during the recovery phase of subacute thyroiditis (inflammation of the thyroid gland).

Levothyroxine is not used to treat obesity or for weight loss.

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. Allergic reactions are usually caused by the inactive ingredients in a product rather than by levothyroxine itself, so your doctor may suggest trying a different brand or form.

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

• sudden pain or trouble moving your hip, wrist, or back;
• fast or irregular heartbeats, or a pounding heartbeat;
• chest pain, pain spreading to your jaw or shoulder;
• wheezing or shortness of breath;
• swelling in your legs or ankles, or new or worsening breathlessness when lying down;
• fever, swollen glands, itching, joint pain, or not feeling well;
• nausea, vomiting, or diarrhea;
• confusion or feeling disoriented;
• a seizure; or
• high blood sugar - increased thirst, increased urination, dry mouth, fruity breath odor.

Common levothyroxine side effects may include:

• fever, hot flashes, increased sweating, feeling too hot;
• tiredness;
• skin rash, flushing, hair loss;
• chest pain, fast or irregular heartbeats, shortness of breath;
• headache, leg cramps, muscle cramps, muscle pain or weakness;
• tremors, feeling nervous, anxious, irritable, or emotional, sleep problems (insomnia);
• increased or change in appetite;
• weight loss or weight gain;
• changes in your menstrual periods, or difficulty getting pregnant; or
• stomach cramps, vomiting, diarrhea.

Most of these side effects happen when the levothyroxine dose is higher than your body needs, which produces symptoms of an overactive thyroid. Tell your doctor if you have any of them so your dose can be checked.

Side effects in children

Children taking levothyroxine may also develop increased pressure around the brain (which can cause headaches, vomiting, or vision changes) or a hip problem called slipped capital femoral epiphysis, which causes hip, groin, or knee pain and limping. Call your child's doctor about any of these symptoms.

Too much levothyroxine in babies can cause the soft spots on the skull to close too early, and in older children it can cause the growth plates to close too early, which may reduce final adult height. Too little levothyroxine can affect a child's learning and growth. This is why children need regular blood tests and check-ups.

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.

Warnings

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.

Levothyroxine has a narrow therapeutic index, which means the difference between too little and too much is small. Taking too much or too little over time can affect your heart, bones, blood sugar, cholesterol, digestion, memory and concentration, fertility, and, in children, growth and development. Your doctor will start you on a suitable dose and adjust it gradually based on blood tests.

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.

If you have an adrenal gland problem, you will need to start treatment with a steroid (glucocorticoid) medicine before you start levothyroxine. Starting levothyroxine first can trigger a sudden, serious drop in adrenal hormones known as an adrenal crisis.

Older adults and people with heart disease are usually started on a lower dose, because too much thyroid hormone can cause an irregular heartbeat (most commonly atrial fibrillation), angina, or other heart problems. Tell your doctor if you develop chest pain, breathlessness, or a fast or irregular heartbeat.

Taking more levothyroxine than your body needs over a long period can weaken your bones and lead to a higher risk of fractures. This is more likely after menopause. Your doctor will aim to give you the lowest dose that controls your thyroid levels.

Oral levothyroxine should not be used to treat myxedema coma. Levothyroxine given into a vein is used instead.

Before taking this medicine

You should not use levothyroxine if you have an untreated or uncontrolled adrenal gland disorder.

You should not use certain liquid or injectable forms of levothyroxine if you are allergic to glycerin or edetate disodium.

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

• a thyroid nodule;
• thyroiditis (inflammation of the thyroid gland);
• heart problems such as a heart attack, stroke, angina, or an irregular heartbeat;
• a blood clot or a blood clotting disorder;
• diabetes (your diabetes medicine may need to be adjusted);
• anemia (low red blood cells);
• weak bones (osteoporosis), or low bone mineral density;
• problems with your pituitary or adrenal gland;
• an allergy to any food or drugs;
• recently received radiation therapy with iodine (such as I-131); or
• kidney disease.

Tell your doctor if you are pregnant or plan to become pregnant. Having hypothyroidism during pregnancy may increase the risk of miscarriage, high blood pressure in pregnancy, pre-eclampsia, stillbirth, or premature birth, and untreated hypothyroidism can affect the baby's brain development. The benefit of treating hypothyroidism outweighs these risks, and levothyroxine should not be stopped during pregnancy.

Your dose needs are often higher during pregnancy. Your doctor will check your thyroid blood tests as soon as your pregnancy is confirmed and at least once in each trimester, and will usually put you back on your pre-pregnancy dose straight after you give birth, with another blood test 4 to 8 weeks later.

Tell your doctor if you are breastfeeding. Levothyroxine passes into breast milk, but no harmful effects on breastfed babies have been reported. Treating hypothyroidism properly may also help milk supply if it is low. Your dose needs may be different while you are nursing.

You may be more likely to have a broken bone while using levothyroxine, particularly if your dose is higher than you need. 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.

• Keep using this medicine even if you feel well. It may take 4 to 6 weeks to get the full benefit of a dose.
• You will need frequent medical tests, and your next dose may be delayed based on the results.
• This medicine can affect the results of certain medical tests. Tell any doctor who treats you that you are using this medicine.
• Tell your doctor if you have a planned surgery or dental procedure.
• Taking more than your recommended dose will not make this medicine more effective, and may cause serious side effects.

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. A full glass of water is important to avoid choking or gagging, because the tablet can start to break apart quickly. Take the medicine at the same time each day.

• Swallow the capsule whole and do not crush, chew, break, or open it. Tell your doctor if your child cannot swallow a capsule whole.
• If you cannot swallow a tablet whole, crush the tablet and mix it with 1 or 2 teaspoons of water; give the mixture right away by spoon or dropper and make sure all of it is taken. Do not save it for later use. Do not mix it into soy-based infant formula or other foods that can stop levothyroxine being absorbed.
• Keep each tablet or capsule in the blister pack until you are ready to take one.
• Store at room temperature away from moisture, heat, and light.

Oral solution (liquid medicine)

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

• Dosages are based on weight in children and teenagers. Your child's dose may change if the child gains or loses weight.
• Your dose needs may change if you switch to a different brand, strength, or form of levothyroxine. Avoid medication errors by using only the medicine your doctor prescribes.
• Tirosint-Sol oral solution comes in single-dose ampules and can be administered 15 minutes before eating breakfast. Squeeze the liquid directly into your mouth, onto a spoon, or into a glass of water to drink right away. Use all ampules within 3 months after opening the foil pouch.
• Ermeza and Thyquidity liquids should be taken on an empty stomach, at least 30 to 60 minutes before breakfast. Store them in their original bottle. Use Ermeza within 90 days and Thyquidity within 8 weeks of opening the bottle.
• Store at room temperature away from moisture, heat, and light.

Intravenous levothyroxine

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

Blood tests, biotin, and monitoring

Your doctor will use blood tests to check that your dose is right, usually 6 to 8 weeks after any dose change, and then every 6 to 12 months once your dose is stable. Children are tested more often, including 2 and 4 weeks after starting treatment and 2 weeks after any dose change.

Tell your doctor or pharmacist if you take biotin, or a multivitamin, hair, skin, or nail supplement that contains biotin (sometimes listed as vitamin B7). Biotin can interfere with thyroid blood tests and give a falsely high or falsely low result, which could lead to the wrong dose change. Stop biotin and any biotin-containing supplements at least 2 days before a thyroid blood test.

If your symptoms and blood tests still suggest an underactive thyroid even though you are on a dose that should be enough, it may be because the medicine is not being absorbed properly, doses are being missed, or another medicine is interfering. Talk to your doctor before your dose is increased.

Related/similar drugs

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, confusion, chest pain, shortness of breath, fast or pounding heartbeats, stroke, seizures, shock, and coma. Symptoms may not appear straight away and can take several days to develop, so get medical advice even if the person seems well.

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, cottonseed meal, walnuts, and high-fiber foods. Grapefruit juice can both delay absorption and reduce the amount of levothyroxine that gets into your body.

If you eat or drink any of these regularly, tell your doctor, as your dose may need adjusting.

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:

• calcium carbonate (Alka-Mints, Caltrate, Os-Cal, Oyster Shell Calcium, Rolaids Soft Chew, Tums, and others);
• sevelamer, lanthanum;
• cholestyramine, colesevelam, colestipol;
• iron supplements, including ferrous sulfate;
• sucralfate;
• sodium polystyrene sulfonate (Kalexate, Kayexalate, Kionex);
• stomach acid reducers - esomeprazole, lansoprazole, omeprazole, rabeprazole, Nexium, Prilosec, Prevacid, Protonix, Zegerid, and others (this applies to most brands of levothyroxine except for Tirosint-SOL); or
• antacids that contain aluminum or magnesium - Gaviscon, Maalox, Milk of Magnesia, Mintox, Mylanta, Pepcid Complete, and others.

Tell your doctor about all your other medicines, especially:

• orlistat;
• phenobarbital, rifampin;
• ketamine, steroid medicines such as dexamethasone;
• antidepressants such as amitriptyline, maprotiline, or sertraline;
• heart or blood pressure medication, including amiodarone, digoxin, propranolol, and other beta blockers;
• furosemide given by injection;
• seizure medicines such as carbamazepine or phenytoin;
• high-dose aspirin, or anti-inflammatory painkillers;
• estrogen, birth control pills, or tamoxifen;
• testosterone or anabolic steroids;
• diabetes medications, including insulin;
• cancer medicine such as imatinib;
• biotin or supplements that contain biotin; or
• a blood thinner - warfarin, Coumadin, Jantoven.

If you have diabetes, levothyroxine can raise your blood sugar, and you may need more of your diabetes medicine or insulin. Check your blood sugar more often when you start, change, or stop levothyroxine, and tell your doctor about any changes.

If you take a blood thinner such as warfarin, levothyroxine can increase its effect. You may need more frequent blood clotting tests and a lower blood thinner dose.

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.

Common questions about levothyroxine

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Further information

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

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