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Levothyroxine (synthetic thyroid hormone)

Written by Stephanie Nera, RPh, PharmD · Pharmacology

Updated Jul 01, 2021

This levothyroxine is a synthetic version of the body’s natural thyroid hormone. Doctors prescribe levothyroxine to correct low levels of thyroid hormones or deficiency. Low thyroid levels may be caused by iodine deficiency, underactive thyroid tissue (hypothyroidism). or a resected thyroid gland (thyroidectomy).



What is levothyroxine used for?

  • Benign goiter with normal thyroid (euthyroid) function
  • Prophylaxis for relapse after thyroidectomy
  • Thyroid hormone replacement therapy
  • Suppression therapy in thyroid cancer
  • Correcting low thyroid hormone levels in hyperthyroid patients being treated with anti-thyroid medications

How should I take levothyroxine?

Read the directions on the packaging for complete information. Check the label and expiration date.

For oral dosage forms, swallow it whole without chewing it. If swallowing it is difficult, scored tablets can be divided or crushed and mixed with 5 to 10 mL of water and consumed immediately. Do not open or dissolve capsules. Take it on an empty stomach, 30 minutes to 1 hour before meals.

For parenteral dosage forms, only a licensed healthcare professional should administer it.

How do I store levothyroxine?

This product is best stored at room temperature away from direct light and moisture. To prevent drug damage, you should not store it in the bathroom or the freezer. 

There may be different brands of this drug that may have different storage needs. So, it is important to always check the product package for instructions on storage, or ask your pharmacist. For safety, you should keep all medicines away from children and pets.

You should not flush this product down the toilet or pour them into a drain unless instructed to do so. Additionally, it is important to properly discard this product when it is expired or no longer needed. Consult your pharmacist for more details about how to safely discard your product.

Precautions & warnings

What should I know before using levothyroxine?

Before using this drug, tell your doctor if you are/have:

  • Pregnant or breastfeeding.
  • Taking any other medicines. This includes any prescription, OTC, and herbal remedies.
  • An allergy to any of the ingredients of this product.
  • Any other illnesses, disorders, or medical conditions.

Is it safe during pregnancy or breastfeeding?

Levothyroxine is the preferred treatment of hypothyroidism during pregnancy. Untreated hypothyroidism increases the risk of infertility, spontaneous abortion, stillbirth, prematurity, low birthweight, and impaired brain development. Therefore, this drug is safe to take when necessary.

It passes into breastmilk, however, it is safe and preferred over other thyroid medications.

Side effects

What side effects can occur from levothyroxine?

Like all drugs, this product may have side effects. If they occur, side effects are generally mild and resolve once treatment is finished or the dose is lowered. Some reported side effects include:

  • Leg cramps
  • Muscle weakness
  • Headache
  • Feeling nervous or irritable
  • Diarrhea
  • Skin rash
  • Mild hair loss
  • Fast or irregular heart rate
  • Fever, hot flashes, sweating
  • Sleep problems (insomnia)
  • Changes in your menstrual periods
  • Vomiting
  • Diarrhea
  • Appetite changes
  • Weight  loss

Seek medical attention if you experience any of the following:

  • Thyrotoxic crisis
  • Convulsions
  • Heart arrhythmia
  • Heart failure
  • Coma

However, not everyone experiences these side effects. In addition, some people may experience other side effects. So, if you have any concerns about a side effect, please consult your doctor or pharmacist.


What drugs may interact with levothyroxine?

This medication may interact with other drugs that you are currently taking, which can change how your drug works or increase your risk for serious side effects. 

To avoid any potential drug interactions, you should keep a list of all the drugs you are using (including prescription drugs, nonprescription drugs, and herbal products) and share it with your doctor and pharmacist. 

Drugs with known interactions:

  • Antacids
  • Iron supplements
  • Estrogen (e.g. hormonal contraceptives, postmenopausal hormone replacement therapy)
  • Biotin (vitamin B7)
  • Diabetes medications
  • Anticoagulants (e.g. coumarin-derivatives)
  • Antivirals
  • Phenytoin
  • Orlistat
  • Tyrosine-kinase inhibitors
  • Glucocorticoids
  • Amiodarone
  • Sertraline
  • Barbiturates

If you experience an adverse drug interaction, inform your doctor immediately to reevaluate your treatment plan. Approaches include dose adjustment, drug substitution, or ending therapy.

Does food or alcohol interact with levothyroxine?

This drug may interact with food or alcohol by altering the way the drug works or increase the risk for serious side effects. Avoid consuming soy products while taking this drug. Additionally, it is best to take it on an empty stomach. Please discuss with your doctor or pharmacist any potential food or alcohol interactions before using this drug.

What health conditions may interact with levothyroxine?

This drug may interact with underlying conditions. This interaction may worsen your health condition or alter the way the drug works. Therefore, it is important to always let your doctor and pharmacist know all the health conditions you currently have, especially:

  • Adrenal gland insufficiency (underactive adrenal gland)
  • Heart attack, acute or recent
  • Thyrotoxicosis (overactive thyroid)
  • Adrenal problems
  • Anemia, pernicious
  • Angina (severe chest pain)
  • Blood clotting problems
  • Diabetes
  • Heart or blood vessel disease (e.g., coronary artery disease)
  • Heart rhythm problems (e.g., arrhythmia, atrial fibrillation)
  • Osteoporosis
  • Pituitary gland problems
  • Patients who have trouble swallowing capsules


The information provided is not a substitute for any medical advice. Therefore, you should always consult with your doctor or pharmacist before using any medication.

What is the dose for an adult?


The initial dose of levothyroxine is 12.5 to 50 mcg once a day. The dosage can be increased in 12.5 to 25 mcg/day increments every 2 to 4 weeks.

In older patients or in younger patients with a history of cardiovascular disease, the dosage should be increased in 12.5 to 25 mcg increments every 3 to 6 weeks.

TSH Suppression

The initial dose is 50 mcg once a day. The dosage may be increased in 25 to 50 mcg increments every 2 to 4 weeks. The typical maintenance dose is 100 to 200 mcg orally once a day.

Thyroid Suppression Test

2.6 mcg/kg/day for 7 to 10 days.

Myxedema Coma

Initial dose: 300 to 500 mcg IV bolus administered one time.

What is the dose for a child?


Congenital hypothyroidism:


  • Oral: Give 10 to 15 mcg of levothyroxine per kg of body weight per day; if patient is at risk for development of cardiac failure, begin with a lower dose. In severe cases of hypothyroidism (T4 less than 5 mcg/dL), a higher initial dose of 12 to 17 mcg/kg/day may be considered.
  • IV or IM: 50% to 75% of the oral dose.

0 to 3 months:

  • 10 to 15 mcg per kg of body weight per day; if the infant is at risk for development of cardiac failure use a lower starting dose of approximately 25 mcg/day; if the initial serum T4 is very low (less than 5 mcg/dL) begin treatment at a higher dosage of approximately 50 mcg/day.

3 to 6 months:

  • 8 to 10 mcg/kg or 25 to 50 mcg once per day.

6 to 12 months:

  • 6 to 8 mcg/kg or 50 to 75 mcg once per day.

1 to 5 years:

  • 5 to 6 mcg/kg or 75 to 100 mcg once per day.

6 to 12 years:

  • 4 to 5 mcg/kg or 100 to 125 mcg once per day.

12 years:

  • 2 to 3 mcg/kg or greater than or equal to 150 mcg once per day.

Patients in which growth and puberty are complete: 1.7 mcg/kg once per day.

For chronic or severe hypothyroidism: 25 mcg once per day and increase dosage as needed in increments of 25 mcg every 2 to 4 weeks until the desired effect is achieved.

How is levothyroxine available?

Levothyroxine is available in the following dosage forms and strengths:

  • Tablet: 212.5 mcg, 5 mcg, 50 mcg, 100 mcg, 150 mcg
  • Oral solution: 100 mcg/5 mL
  • Solutions for injection: 100 mcg/5 mL

What should I do in case of an emergency or overdose?

In case of an emergency or an overdose, call your local emergency services or go to the nearest emergency room.

What should I do if I miss a dose?

If you miss a dose, take it as soon as possible. However, if it is almost time for your next dose, skip the missed dose and take your regular dose as scheduled. Do not take a double dose.


Hello Health Group does not provide medical advice, diagnosis or treatment.

Written by

Stephanie Nera, RPh, PharmD


Updated Jul 01, 2021

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