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Boxed warning:

A boxed warning is the highest safety warning issued by the US Food and Drug Administration (FDA) and should be discussed with a doctor before beginning a medication.

The FDA has issued a warning that levothyroxine should not be used for the treatment of obesity nor to promote weight loss.

Taking a standard daily dose of levothyroxine consistent with thyroid regulation therapy will not cause individuals with normal thyroid hormone levels to lose weight. In addition, taking levothyroxine other than intended or in large doses can be toxic and life-threatening. This medication can also be dangerous if mixed with drugs that reduce appetite.¹ ²

If you want to manage your weight or achieve weight loss, speak with your doctor.

What is levothyroxine?

Levothyroxine, also known as levothyroxine sodium, is a synthetic replacement for thyroxine (T4), a hormone naturally produced by the thyroid gland. It’s a medication that works within the genetic material of cells to activate the production of proteins that increase the body’s metabolism.³

Levothyroxine is available in solution, tablet, and capsule formulations for oral administration. The drug is also available in powder and solution formulations for injection that a healthcare professional must administer.

Levothyroxine can be obtained only with a doctor’s prescription. 

In the US, levothyroxine goes by many brand names, including Synthroid, Euthyrox, Levoxyl, LEVO-T, Tirosint, Thyquidity, and Unithroid, among others. It is also available as a generic drug.⁴

What is levothyroxine used to treat?

Levothyroxine is FDA-approved to treat the following conditions in patients of all ages with:

  • Primary hypothyroidism (thyroid gland-specific)

  • Secondary hypothyroidism (pituitary-related)

  • Tertiary hypothyroidism (hypothalamus-related)

    • Congenital

    • Acquired

Thyroid cancer

  • Thyrotropin-dependent, well-differentiated - to suppresses thyroid-stimulating hormone (TSH)

Dosage forms and strengths

Oral tablets

Levothyroxine tablets are available in the following strengths, including generic and branded products (Euthyrox, Levo-T, Levoxyl, Synthroid, Unithroid), which vary in dose:

  • 25mcg, 50mcg, 75mcg, 88 mcg, 100mcg, 112mcg, 125mcg, 137mcg, 150mcg, 175mcg, 200mcg, 300mcg

Oral capsules

Levothyroxine capsules come in a range of strengths, including generic and branded products (generic, Tirosint), which vary in dose:

  • 13mcg, 25mcg, 37.5mcg, 44mcg, 50mcg, 62.5mcg, 75mcg, 88mcg, 100mcg, 112mcg, 125mcg, 137mcg, 150mcg, 175mcg, 200mcg


  • Thyquidity: 20mcg/mL

  • Tirosint: 13mcg/mL, 25mcg/mL, 37.5mcg/mL, 44mcg/mL, 50mcg/mL, 62.5mcg/mL, 75mcg/mL, 88mcg/mL, 100mcg/mL, 112mcg/mL, 125mcg/mL, 137mcg/mL, 150mcg/mL, 175mcg/mL, 200mcg/mL

(There is also an injectable form of levothyroxine for administering only in clinical settings).

How do you take levothyroxine?

How you take levothyroxine will depend on the form, dose, strength, age, condition you are treating, medications you take, and any other medical conditions you may have. 

For best results, take this medication as routinely as you can, ideally at the same time daily.

Levothyroxine is typically taken in the morning on an empty stomach, but always follow your prescribing doctor’s instructions about how and when to take it.

After taking a levothyroxine tablet or capsule, drink a full glass of water and refrain from eating for 30 minutes to an hour.

Avoid foods or supplements with calcium (such as dairy), soybean products, grapefruit juice, or iron for at least 4 hours before and after your dose of levothyroxine.

Unless specified by your doctor, do not take levothyroxine at the same time as drugs for management of heartburn or acid reflux, such as antacids and proton pump inhibitors, because they can decrease absorption of the drug.⁵

Check with your doctor about best practices for other medication dosing times because levothyroxine interacts with numerous other drugs, which can reduce its effectiveness (continue to “Interactions with other drugs” for more information).  

Seeing results 

Levothyroxine is usually taken as an ongoing lifelong treatment for hypothyroidism. It does not cure the underlying condition.

After starting levothyroxine treatment, you may notice improvement in symptoms like lack of energy within several weeks, but it can also take closer to 4–6 weeks to achieve full therapeutic results.⁶

When you replace thyroxine with a synthetic version like levothyroxine, periodic monitoring of TSH is necessary, typically starting at the 6–8 week mark.⁷

Your doctor may adjust your dose if needed and continue to monitor you after 4–6 months and then annually.

If you do not get the expected results from levothyroxine, your doctor may refer you to an endocrinologist (a hormone specialist).

Who should not take levothyroxine?

You should not take levothyroxine if you have had an allergic reaction or intolerance to it, any of its components, or a similar drug.

Levothyroxine is usually unsuitable for those with:

  • Acute heart attack

  • Untreated adrenal insufficiency (e.g., Addison’s disease) 

  • Acute myocarditis (inflammation of one of the heart’s layers)

  • Pancarditis (inflammation of the entire heart)

  • Irregular heartbeat (arrhythmia)

  • Thyrotoxicosis (high levels of circulating thyroid hormone or thyroid storm)

  • Hyperthyroidism (overactive  thyroid)

  • Myxedema coma

Caution should be used when taking levothyroxine if you have cardiovascular disease, live with diabetes (due to levothyroxine’s potential to increase blood sugar levels), osteoporosis, or when giving this medication to an infant, child, or older adult.⁸

Potential side effects of levothyroxine

Common side effects

Levothyroxine’s common side effects can include one or more of the following:

  • Increased appetite

  • Palpitations or rapid heart rate

  • Stomach cramps

  • Weight loss

  • Heat intolerance

  • Hair loss (usually temporary)

  • Excessive sweating

  • Hyperactivity

  • Tremors

  • Nervousness and anxiety

  • Irritability

  • Mood swings

  • Trouble sleeping

  • Muscle weakness

  • Headache

  • Menstrual changes

  • Diarrhea

  • Heat sensitivity

  • Vomiting

  • Leg cramps or muscle spasms

  • Joint pains

  • Flushing

  • Rashes

These effects may go away after a few days or weeks if mild. However, talk to your doctor if they are severe or continue for longer than a few days.

Severe side effects

Call 911 immediately if you experience any severe side effects, which may include the following:⁹

  • Angina (chest pain, discomfort in your upper body, shortness of breath)

  • Heart attack

  • Heart failure (shortness of breath, swelling in your legs, ankles, or feet, extreme tiredness)

  • Irregular heart rhythm 

  • Rapid heart rate

  • High blood pressure

  • Difficulty breathing

  • Seizures

This list is not exhaustive and cannot account for your unique medical history. Discuss possible side effects (and how to manage them if they occur) with your prescribing doctor.


If you think that you or someone else has taken too much levothyroxine, call 911 or visit the nearest emergency department.

Levothyroxine poisoning is uncommon, but the risk is notably high for children who accidentally take the medicine or older adults who take too high a dose. In addition, there isn’t a specific antidote to reverse this drug overdose. Instead, treatment typically entails various medical measures based on the time needed to receive medical intervention after the ingestion.¹⁰

Overdose symptoms are often a more severe and persistent form of the reported side effects.

Common signs of toxicity might include, but may not be limited to:

  • Shakiness

  • Elevated heart rate

  • Abnormal heart rhythms

  • High blood pressure

  • Diarrhea

  • Anxiety

More rarely, signs of overdose involve:

  • Convulsions

  • Thyroid storm

  • Acute psychosis

  • Arrhythmias

  • Heart attack 

Levothyroxine allergy information

Allergic reactions to levothyroxine have been reported. Symptoms range from mild to severe and may include any of the following:

  • A skin rash with red, swollen, blistered, itchy, or peeling skin

  • Hives

  • Trouble breathing or talking, wheezing

  • Swelling in the mouth, face, lips, or throat

  • Tightness in the chest or throat

  • Rapid heart rate

  • Vomiting

  • Loss of consciousness

Call 911 or visit the nearest emergency department if you experience severe signs of a drug allergy while taking levothyroxine.

Long-term use of levothyroxine

Levothyroxine is widely used and generally considered a safe, effective drug for the long-term treatment of hypothyroid conditions.

However, some researchers have raised concerns about a relationship between elevated cancer risk and long-term use of this medication. 

A 2021 study found a connection between using levothyroxine and increased brain, skin, prostate, pancreatic, female breast, lung, and colorectal cancers. It’s essential to be clear that though a correlation was found, researchers did not conclude that this medication causes cancer. The researchers also acknowledged multiple limitations in the study's design, concluding that continued, rigorous research is called for.¹¹

A different 2010 retrospective case-control study found that levothyroxine is associated with a reduced risk of colorectal cancer.¹²

However, long-term and untreated hypothyroidism has many risks to health, including cardiovascular disease, heart failure, abnormal glucose metabolism, obesity, impaired cognitive function, kidney dysfunction, and myxedema coma.¹³

To minimize risks and avoid overtreating hypothyroidism, your prescribing doctor will use lab tests to monitor your condition, determine an effective maintenance dose, and adjust as needed. 

Pregnancy category

Levothyroxine has been designated a pregnancy category A medication by the FDA, which means that in human studies, patients taking it during pregnancy have not shown an increased risk of congenital abnormalities.¹⁴

Levothyroxine and pregnancy


Levothyroxine is considered generally safe during pregnancy, but your doctor should prescribe and closely monitor any medication you take.

Untreated hypothyroidism poses substantial risks to the pregnancy. Potential complications include hypertension and pre-eclampsia, premature delivery, fetal cognitive difficulties, and stillbirth.

If you are already on levothyroxine, it's important to continue taking regularly scheduled doses throughout pregnancy.  Being pregnant can increase your levothyroxine requirements, and your doctor may recommend a change in your daily dosage. You'll need regular thyroid hormone blood levels assessed to determine whether you're taking the correct dose of levothyroxine. Abnormal levels can cause problems for you and your baby.¹⁵


You don't need to stop breastfeeding if you're taking levothyroxine unless recommended by your physician. Thyroid hormones are typically found in breast milk, whether made naturally by the body or taken in a synthetic form.

If breastfeeding, you need to continue taking your prescribed levothyroxine, which replaces what your body would usually make. Your body needs adequate levels of thyroid hormones to produce enough breast milk for your baby. Like with pregnancy, your requirement for thyroid medication will likely increase.¹⁶

If you notice that your baby is not feeding as well as usual, or if you have any other concerns about breastfeeding, talk to your baby’s healthcare clinician as soon as possible.

Missed doses

If you realize you have missed a dose of levothyroxine within a few hours of when you usually take it, take the missed dose immediately to help maintain a consistent thyroid hormone level.

If it's closer to the time for your next scheduled dose, skip the missed dose and continue with your regular dosing schedule. 

Do not double up on doses to make up for missed ones.

Interactions with other drugs

Some medications may interfere with the absorption of thyroid hormones, while others increase or decrease levothyroxine levels in the body.

Here are some categories of medications that can impact the absorption of levothyroxine:¹⁷ ¹⁸

  • Acid-blocking medications (over-the-counter and prescription)

    • Calcium-containing drugs, such as calcium carbonate (Tums, Rolaids, Alka-Seltzer)

    • Magnesium-containing drugs, such as magnesium hydroxide (Mylanta, Gelusil)

    • H2 blockers like famotidine (Pepcid, Zantac 360), cimetidine (Tagamet HB 200), nizatidine

    • Proton pump inhibitors, such as omeprazole (Prilosec), esomeprazole (Nexium), lansoprazole (Prevacid)

    • Sucralfate (Carafate), vonoprazan (Voquezna)

  • Antivirals such as efavirenz, ritonavir (Norvir), tipranavir (Aptivus)

  • Antidepressants like sertraline (Zoloft) 

  • Blood thinners such as warfarin (Coumadin)

  • Calcium supplements

  • Cardiac medications like digoxin (Lanoxin)

  • Cholesterol-lowering drugs such as colestipol (Colestid), colesevelam (Welchol)

  • Diabetes medicines such as alogliptin (Nesina), bexagliflozin (Brenzavvy), dulaglutide (Trulicity), glyburide, glipizide (Glucotrol XL), linagliptin (Tradjenta), insulin, metformin (Glumetza, Riomet), semaglutide (Ozempic), tirzepatide (Mounjaro), and others

  • Diuretics like furosemide (Lasix)

  • Estrogen hormone (for contraception or hormone replacement), including estradiol (Estrace), ethinyl estradiol (Cyclessa, Kariva, and others)

  • Iron supplements such as ferrous sulfate, ferrous gluconate, ferrous fumarate

  • Lithium

  • Magnesium supplements

  • Seizure medications like carbamazepine (Tegretol, Equetro), phenytoin, phenobarbital, valproic acid

  • Weight-management drugs such as orlistat (Xenical)

These lists do not include every possible drug that may interact with levothyroxine, so review all medications you take with your prescribing doctor.

Can I drink alcohol while taking levothyroxine?

Drinking alcohol while taking levothyroxine doesn’t change this medication’s effectiveness, and they don’t interact.

However, alcohol consumption could increase levothyroxine’s known side effects of nausea and dehydration, and it’s important to maintain proper hydration for thyroid function.

If you drink alcohol, ask your doctor whether you can safely consume it while taking levothyroxine.

What to discuss with your doctor before you start taking levothyroxine?

Closely review the products you use daily or occasionally before starting levothyroxine:

For instance:

  • Any prescription and nonprescription medication, vitamins, nutritional supplements, and herbal products you are taking (and how frequently you take them)

  • Usage of alcohol or substances 

Review your health history, and be sure to mention:

  • Any allergies to medications and what reactions occurred

  • Any concurrent conditions you have, such as diabetes, heart disease, heart failure, abnormal heart rhythm, bleeding problems, osteoporosis or bone disease, liver or kidney disease

  • If you have adrenal or pituitary gland insufficiency

  • If you have previously had thyroid surgery

  • If you have recently had radiation therapy

  • If you are pregnant, breastfeeding, or plan to become pregnant

  • Any upcoming surgeries or dental procedures

Stopping levothyroxine

Speak to your doctor before discontinuing this medication. 

Stopping levothyroxine could result in a return of your hypothyroid symptoms, such as

  • Weight gain

  • Fatigue

  • Difficulty tolerating cold

  • Thinning hair

  • Dry skin

  • Constipation

Tips for taking levothyroxine

  • Take levothyroxine at least 4 hours apart from any heartburn medication, calcium, magnesium, iron supplements, or multivitamins.

  • When taken on an empty stomach (about 30 minutes to one hour before breakfast), levothyroxine absorption increases.

  • Most of the adverse effects of taking levothyroxine correspond with hyperthyroidism symptoms, which may mean your dose is too high. Talk to your doctor if you experience any of the listed side effects. Your thyroid levels will need evaluation, and your dosage may need to be adjusted.

  • Switching from one brand of levothyroxine to another can affect the drug’s absorption and cause abnormal thyroid hormone levels in your blood. It’s helpful to fill the prescription at the same pharmacy every time and pay attention to the drug manufacturer. You may not be able to change the manufacturer contracted with your pharmacy, but you can talk with your prescriber about changing your dosage or pharmacy if needed.¹⁹

  • Ensure you have informed your doctor of any medical conditions or pregnancy because they will need to monitor you more closely and adjust your dosage.

  • Unexpectedly high blood pressure may be an early warning sign that your levothyroxine dosage is too high. If you have a home blood pressure cuff, monitor your blood pressure regularly. Your nearest pharmacy might have a public blood pressure kiosk machine available. Alternatively, you could arrange to have it checked at your doctor’s office (particularly if they change your levothyroxine dosage).

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View related condition trials page

Here at HealthMatch, we’ve done our best to ensure that the information provided in this article is helpful, up to date, and, most importantly, accurate.

However, we can’t replace the one-to-one advice of a qualified medical practitioner or outline all of the possible risks associated with this particular drug and your circumstances.

It is therefore important for you to note that the information contained in this article does not constitute professional medical or healthcare advice, diagnosis or recommendation of treatment and is not intended to, nor should be used to, replace professional medical advice. This article may not always be up to date and is not exhaustive of all of the risks and considerations relevant to this particular drug. In no circumstances should this article be relied upon without independent consideration and confirmation by a qualified medical practitioner.

Your doctor will be able to explain all possible uses, dosages, precautions, interactions with other drugs, and other potential adverse effects, and you should always talk to them about any kind of medication you are taking, thinking about taking or wanting to stop taking.

Curious about clinical trials?

Access the latest treatments and medications. unavailable elsewhere - entirely free of charge. We make it easy to take part.