What is levothyroxine sodium, and how does it work (mechanism of action)?

Levothyroxine is a synthetic (man-made) version of the principle thyroid hormone, thyroxine (T4) that is made and released by the thyroid gland. Thyroid hormone increases the metabolic rate of cells of all tissues in the body. In the fetus and newborn, thyroid hormone is important for the growth and development of all tissues including bones and the brain. In adults, thyroid hormone helps to maintain brain function, utilization of food, and body temperature, among other effects.

What brand names are available for levothyroxine sodium?

Synthroid, Levoxyl, Levothroid, Unithroid, Tirosint, Levo-T (discontinued brand)

Is levothyroxine sodium available as a generic drug?

GENERIC AVAILABLE: Yes. Generic and branded tablets of levothyroxine may differ:

  • in the amount of levothyroxine they contain,
  • the absorption of the levothyroxine into the body, and
  • the distribution of levothyroxine throughout the body.

This means that ingestion of 1 mg of generic levothyroxine may not have the same effect on the body as 1 mg of another generic or branded levothyroxine. Practically speaking, this means that when changing between levothyroxine manufactured by different pharmaceutical companies, a change in dose may be necessary to maintain the desired effect or to prevent toxicity. When switching between brands or generics, it is important to ensure that both preparations are equivalent or to check blood thyroid levels weekly.

Do I need a prescription for levothyroxine sodium?


What are the side effects of levothyroxine sodium?

Levothyroxine therapy usually is well-tolerated. If symptoms occur, they often are due to toxic levels of thyroid hormone, and the symptoms are those of hyperthyroidism.

The most commonly reported side effects include:

What is the dosage for levothyroxine sodium?

For adult hypothyroidism, levothyroxine is started at 12.5-125 mcg/day administered orally. Starting doses and dose changes may differ with individual patients based upon age, the presence of cardiovascular disease, the development of tolerance (reduced effectiveness with continued use), side effects to the medication, and blood levels of thyroid hormone. It may take one to three weeks after initiating therapy with levothyroxine or changing the dose before effects are seen. The goal of replacement therapy is to maintain a normal blood thyroxine level.

Which drugs or supplements interact with levothyroxine sodium?

Initiation or discontinuation of therapy with levothyroxine in diabetic patients may create a need for an increase or decrease in the required dose of insulin and/or antidiabetic drug, (for example, glyburide [Micronase]).

Levothyroxine may increase the effect of blood thinners such as warfarin (Coumadin). Therefore, monitoring of blood clotting is necessary, and a decrease in the dose of warfarin may be necessary.

Intravenous administration of epinephrine to patients with coronary artery disease may lead to complications ranging from difficulty in breathing to a heart attack. These complications may occur more frequently among patients also taking levothyroxine. Therefore, careful observation is needed when intravenous epinephrine is given to patients receiving levothyroxine who also have coronary artery disease.

Converting a state of hypothyroidism (under activity) to a normal state (euthyroid state) with levothyroxine may decrease the actions of certain beta-blocking drugs, (for example, metoprolol [Lopressor] or propranolol [Inderal]). It may be necessary, therefore, to change the dose of beta-blocker. For the same reason, the dose of digoxin (Lanoxin), a drug used to manage heart failure or an irregular heart rhythm (for example, atrial fibrillation), also may need to be changed.

Converting hypothyroidism to the euthyroid state with levothyroxine may increase the blood level of theophylline (Slo-Bid), and it may be necessary to change the dose of theophylline.

Taking levothyroxine at the same time as calcium carbonate, ferrous sulfate, cholestyramine (Questran) or colestipol (Colestid) may decrease the effect of levothyroxine and lead to hypothyroidism. This occurs because the levothyroxine binds to these drugs and is not absorbed. Taking the levothyroxine one hour before or four hours after these drugs is necessary to prevent binding.

Is levothyroxine sodium safe to take if I’m pregnant or breastfeeding?

Thyroid hormone therapy in nursing mothers is usually safe but should be supervised by a physician.

What else should I know about levothyroxine sodium?

What preparations of levothyroxine sodium are available?
  • Tablets: 0.025, 0.05, 0.075, 0.088, 0.1, 0.112, 0.125, 0.137, 0.15, 0.175, 0.2 and 0.3 mg.
  • Capsules: 0.013, 0.025, 0.05, 0.075, 0.088, 0.1, 0.125 and 0.15 mg.
  • Powder for intravenous injection: 200 mcg and 500 mcg per vial.
How should I keep levothyroxine sodium stored?

Levothyroxine tablets, capsules, and dry powder should be kept at room temperature, 15 C to 30 C (59 F to 86 F) in a light-resistant, tight container.

Powdered levothyroxine for intravenous injection should be used immediately once mixed with liquid.

Medically Reviewed on 11/5/2018


Medically reviewed by John P. Cunha, DO, FACOEP; Board Certified Emergency Medicine


FDA Prescribing Information