Key takeaways

  • Levothyroxine oral tablet is a prescription drug used to treat hypothyroidism, some types of thyroid cancer, and goiter in certain situations.
  • Generic levothyroxine comes in three forms: an oral tablet, an oral capsule, and an injectable solution that’s only given by a healthcare professional.
  • Levothyroxine oral tablet is also available as the brand-name drugs Levoxyl, Synthroid, and Unithroid.

Specifically, the Food and Drug Administration (FDA) has approved levothyroxine oral tablet to treat the following conditions in certain situations:

Levothyroxine is a generic drug. Levothyroxine oral tablet is also available as the brand-name drugs Levoxyl, Synthroid, and Unithroid. A generic medication contains an exact copy of the active ingredient in a brand-name medication. A generic is considered as safe and effective as the original drug but tends to cost less.

If you’re interested in treatment with one of its brand-name versions instead of levothyroxine, talk with your doctor. They can tell you whether the drug you’re interested in comes in forms and strengths that can be used for your condition. If you have insurance, you’ll also need to check whether your plan will cover that particular drug.

Levothyroxine oral tablets may be used as part of combination therapy. (This means you may need to take it with other medications.)

Note: This article focuses on the oral tablet form of levothyroxine, unless otherwise specified. If you have questions about the oral capsule or injectable solution forms of this medication, talk with your doctor or pharmacist.

Levothyroxine has a boxed warning. A boxed warning is the most serious warning from the FDA. For details, see the “Warnings” section.

How it works

Levothyroxine belongs to a class of drugs called hormones. It works by providing the thyroid hormone that your thyroid gland would produce if it were working as it should.

Your doctor will order blood tests to monitor your thyroid hormone level during your levothyroxine treatment. The tests will indicate whether the drug is effectively treating your condition. You should also have more energy and less weakness once the drug starts working.

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Levothyroxine oral tablet can cause side effects. These lists do not include all possible side effects of this medication. To learn more, check out the drug’s prescribing information, or talk with your doctor or pharmacist.

More common side effects

The more common side effects reported with levothyroxine include:

If these effects are mild, they may go away within a few days to a couple of weeks. If they’re more severe or don’t go away, talk with your doctor or pharmacist.

Serious side effects

Serious side effects from levothyroxine aren’t common, but they can occur. Call your doctor right away if you have serious side effects. But if your symptoms feel 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 with levothyroxine include:

Note: After the Food and Drug Administration (FDA) approves a drug, it tracks and reviews side effects of the medication. Sharing your experience helps make medications safer for everyone by giving doctors and researchers more information about how the drug works in real life. If you develop a side effect while taking levothyroxine and want to tell the FDA about it, visit MedWatch or call 800-FDA-1088.

Levothyroxine oral tablet can interact with other medications, herbs, vitamins, or supplements. An interaction is when a substance changes the way a drug works. This can be harmful or prevent the drug from working well.

Be sure to tell your doctor and pharmacist about all drugs, supplements, vitamins, and herbs you’re taking. Sharing this information can help you avoid potential interactions.

Examples of drugs that can cause interactions with levothyroxine are listed here. Note that these lists do not contain all drugs that may interact with levothyroxine. If you have questions about drug interactions that may affect you, ask your doctor or pharmacist.

Interactions that increase the risk of side effects

Taking levothyroxine with certain drugs may result in an increase in side effects. Examples of these drugs include:

  • Antidepressants, such as amitriptyline and maprotiline: The side effects of both the antidepressant and levothyroxine may increase when you take these drugs together. This may put you at risk for irregular heart rhythms.
  • Sympathomimetic drugs, such as pseudoephedrine and albuterol: The effects of both the sympathomimetic drug and levothyroxine may increase when you take these drugs together. This may put you at risk of serious heart problems.
  • Blood thinners, such as warfarin: Taking blood thinners with levothyroxine may increase your risk of bleeding. Your doctor may need to decrease the dosage of your blood thinner if you’re also taking levothyroxine.
  • Ketamine: Taking ketamine with levothyroxine may increase your risk of high blood pressure and fast heart rate.

Interactions that can make drugs less effective

When you take levothyroxine with certain drugs, the amount of levothyroxine in your body may be decreased and may not work as well to treat your condition.

Examples of drugs that can affect how well levothyroxine works:

  • Amiodarone
  • Beta-blockers
  • Calcium carbonate or ferrous sulfate: Take levothyroxine at least 4 hours before or after taking either of these medications to help ensure the levothyroxine works properly.
  • Cancer drugs that belong to the tyrosine-kinase inhibitors class, such as imatinib
  • Colesevelam, cholestyramine, colestipol, kayexalate, or sevelamer: Take levothyroxine at least 4 hours before taking any of these medications to help ensure the levothyroxine works properly.
  • Glucocorticoids
  • Orlistat
  • Rifampin and anti-seizure drugs, such as carbamazepine and phenobarbital
  • Sertraline (Zoloft): If you take sertraline with levothyroxine, your doctor might need to increase your levothyroxine dosage to keep it working well for you.
  • Simethicone and antacids, such as aluminum or magnesium

When certain drugs are used with levothyroxine, they may not work as well. This is because the amount of these drugs in your body may be decreased.

Examples of drugs that may not be as effective when taken with levothyroxine:

  • Diabetes drugs: If you take diabetes drugs such as insulin, metformin, nateglinide, glipizide, or pioglitazone with levothyroxine, your doctor may need to increase your dose of the diabetes drug(s).
  • Digoxin: If you take this drug with levothyroxine, your doctor may need to increase your dose of digoxin.
  • Theophylline: Your doctor may monitor the levels of theophylline in your body if you take it with levothyroxine.

All possible dosages and drug forms may not be included here. Your dosage, drug form, and how often you take the drug will depend on:

  • your age
  • the condition you’re taking the drug to treat
  • how severe your condition is
  • how you react to the first dose
  • other medical conditions you have

Drug forms and strengths

The following drug strengths are shown in micrograms (mcg).

Generic: levothyroxine

  • Form: oral tablet
  • Strengths: 25 mcg, 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg, 175 mcg, 200 mcg, 300 mcg

Brand: Levoxyl

  • Form: oral tablet
  • Strengths: 25 mcg, 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg, 175 mcg, 200 mcg

Brand: Synthroid

  • Form: oral tablet
  • Strengths: 25 mcg, 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg, 175 mcg, 200 mcg, 300 mcg

Brand: Unithroid

  • Form: oral tablet
  • Strengths: 25 mcg, 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg, 175 mcg, 200 mcg, 300 mcg

Dosage for hypothyroidism

Adult dosage (ages 18 to 50 years):

  • Your dosage will be based on several factors, including your age, weight, thyroid hormone levels, other conditions you have, and other medications you take. The typical daily dosage is 1.6 mcg per kilogram (kg) of body weight. (For reference, 1 kg equals approximately 2.2 pounds [lb].)
  • Dosages are usually less than 200 mcg daily.

Child dosage (ages 0 to 18 years):

Pediatric dosing is based on the child’s weight and their thyroid hormone levels, ranging from 10 to 15 mcg/kg/day for infants ages 0 to 3 months to 1.6 mcg/kg/day for adolescents who have completed puberty.

Senior dosage (ages 50 years and older):

  • The typical starting dosage is 12.5 to 25 mcg/day.
  • Your doctor will adjust your dosage every 6 to 8 weeks, based on your thyroid hormone lab results.

Dosage for goiter

Adult dosage (ages 18 years and older):

Your dosage is based on your needs and thyroid hormone levels. Your doctor will determine the right dosage for you and adjust it based on your current level of thyroid hormone.

Child dosage (ages 0 to 18 years):

There are no dosage recommendations for people younger than 18 years.

Dosage for thyroid cancer

Adult dosage (ages 18 years and older):

Your dosage is based on your needs and thyroid hormone levels. Your doctor will determine the right dosage for you and adjust it based on your current level of thyroid hormone.

Child dosage (ages 0 to 18 years):

There are no dosage recommendations for people younger than 18 years.

Dosage for special populations

For pregnant women: You may need a higher dosage of levothyroxine.

For people with heart disease:

  • The recommended starting dosage is 12.5 to 25 mcg/day.
  • Your dosage may be adjusted every 6 to 8 weeks.

How to take this medication

  • Take levothyroxine without food, on an empty stomach.
  • Take the tablet with a full glass of water to avoid choking or gagging.
  • Take levothyroxine in the morning. Take it 30 minutes to 1 hour before your first meal of the day.
  • For children who cannot swallow tablets whole, you can cut or crush the tablet and mix it with 5 to 10 mL of water (about 1 to 2 teaspoons). They should take this mixture immediately.

Note: If you have difficulty swallowing tablets, ask your doctor or pharmacist for suggestions, or check out these tips for swallowing pills.

Missed dose

If you miss a dose of levothyroxine, take it as soon as you remember. But if you remember just a few hours before your next scheduled dose, take only one dose. Never try to catch up by taking two doses at once. This could result in dangerous side effects.

This section describes the warnings and precautions for levothyroxine.

Boxed warning

This drug has a boxed warning. This is the most serious warning from the Food and Drug Administration (FDA). A boxed warning alerts doctors and patients about drug effects that may be dangerous.

Not for obesity or weight loss: Thyroid hormones, including levothyroxine, should not be used for weight loss or to treat obesity. Taking doses that are higher than recommended can lead to serious or even life threatening effects.

Recall warning

Certain brands and strengths of medication for hypothyroidism (underactive thyroid) have been recalled. This is due to the medication having less active drug (levothyroxine) than it’s supposed to. Taking these tablets can lead to symptoms of hypothyroidism, such as fatigue, constipation, hair loss, and depression. Reduced effectiveness can be particularly dangerous for older adults and those who are pregnant.

Call your pharmacist to find out if your medication has been recalled. Do not stop taking the medication unless advised to do so by your healthcare professional.

Vitamin supplements and antacids warning

Taking calcium, iron supplements or antacids can decrease the amount of levothyroxine your body absorbs. Do not take levothyroxine within 4 hours of taking these supplements or antacids.

Allergy warning

Levothyroxine can cause a severe allergic reaction. Symptoms can include:

If you develop these symptoms, call 911 or go to the nearest emergency room.

Do not take this drug again if you’ve ever had an allergic reaction to it. Taking it again could be fatal (cause death).

Food interactions warning

Certain foods, such as soybean flour, cotton seed meal, walnuts, and other dietary fibers, may affect how well your body absorbs levothyroxine.

Talk with your doctor or pharmacist if you plan to eat these foods during your treatment with levothyroxine. You may need dose adjustments if you regularly take the drug within 1 hour of any food that affects absorption.

Warnings for people with certain health conditions

Before starting levothyroxine, discuss your health history with your doctor. This drug may not be right for you if you have certain medical conditions or other factors affecting your health.

Be sure to talk with your doctor if any of the following apply to you:

Note: Oral levothyroxine should not be used to treat myxedema coma, as this condition requires intravenous (IV) infusion administration for effective treatment.

Warnings for other groups

For pregnant women: Studies of levothyroxine in pregnant women haven’t shown risk to the fetus. Talk with your doctor if you’re pregnant or planning to become pregnant. It appears unlikely that this drug will harm a pregnancy.

Not treating hypothyroidism could cause problems for both you and your pregnancy. You should not stop taking this drug during pregnancy.

For women who are breastfeeding: Small amounts of levothyroxine may pass into breast milk, but this drug is usually safe to take while breastfeeding. Talk with your doctor about the best way to feed your child while taking levothyroxine.

For seniors: If you’re older than 65 years, you may have a higher risk of developing negative heart effects (such as irregular heart rhythms) while taking this drug. Your doctor may choose to start you on a lower dosage.

For children: Levothyroxine has only been approved for use in children for the treatment of hypothyroidism. The tablet can be used safely in children of all ages.

As with all medications, the cost of levothyroxine can vary. The actual price you’ll pay depends on your insurance plan, your location, and the pharmacy you use.

Levothyroxine oral tablet is used for long-term treatment. It comes with serious risks if you do not take it as prescribed.

If you stop taking the drug or don’t take it at all: Your thyroid hormones will remain low, which may result in low energy levels, tiredness, weakness, slower speech, constipation, or thickened skin. It may even lead to coma.

If you miss doses or don’t take the drug on schedule: Your medication may not work as well or may stop working completely. For this drug to work well, a certain amount needs to be in your body at all times.

If you take too much: You could have dangerous levels of the drug in your body. Symptoms of an overdose can include:

  • confusion
  • disorientation
  • stroke
  • shock
  • coma

If you think you’ve taken too much of this drug, call your doctor. You can also call America’s Poison Centers at 800-222-1222 or use its online tool. However, if your symptoms are severe, call 911 or your local emergency number or go to the nearest emergency room right away.

Keep these considerations in mind if your doctor prescribes levothyroxine for you.

Storage

  • Store levothyroxine at room temperature between 59°F and 86°F (15°C and 30°C).
  • Keep it away from light.
  • Don’t store this medication in moist or damp areas, such as bathrooms.

Travel

When traveling with your medication:

  • Always carry your medication with you. When flying, never put it into a checked bag. Keep it in your carry-on bag.
  • Don’t worry about airport X-ray machines. They can’t harm your medication.
  • You may need to show airport staff the pharmacy label for your medication. Always carry the original prescription-labeled container with you.
  • Don’t put this medication in your car’s glove compartment or leave it in the car. Be sure to avoid doing this when the weather is very hot or very cold.

Prior authorization

Many insurance companies require a prior authorization for certain brand-name forms of this drug. This means your doctor will need to get approval from your insurance company before your insurance company will pay for the prescription.

There are other drugs available that could treat your condition. These include:

  • liothyronine (Cytomel)
  • liotrix (Thyrolar)
  • natural desiccated thyroid (NDT) extracts, such as:

Some treatments may be better suited for you than others. Talk with your doctor about drug options that may work for you.

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 other 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.