Medically Reviewed on 5/10/2023

Generic Name: methimazole

Brand Names: Tapazole, Northyx (discontinued)

Drug Class: Antithyroid Agents

What is methimazole, and what is it used for?

Methimazole is used to treat an overactive thyroid gland (hyperthyroidism). It also is used for decreasing symptoms of hyperthyroidism in preparation for surgical removal of the thyroid gland or before inactivating the thyroid gland with radioactive iodine. Long-term use of methimazole may lead to a remission of the hyperthyroidism.

Grave's disease is the most common cause of hyperthyroidism. Grave's disease is an autoimmune disease resulting from antibodies that attach to receptors on thyroid hormone-producing cells in the thyroid gland and trigger overproduction of thyroid hormone. An enzyme (peroxidase) produces thyroid hormones, i.e., thyroxine (T4) and triiodothyronine (T3), by combining iodine with a protein called thyroglobulin. Methimazole prevents iodine and peroxidase from their normal interactions with thyroglobulin to form T4 and T3. This action decreases thyroid hormone production. Methimazole also interferes with the conversion of T4 to T3. Since T3 is more potent than T4, this also reduces the activity of thyroid hormones. The FDA approved methimazole in March 1999.

What are the side effects of methimazole?

Methimazole is generally well-tolerated with side effects occurring in 3 out of every 100 patients. The most common side effects are related to the skin and include:

Other common side effects are:

Less common but more serious side effects include a decrease in white blood cells (agranulocytosis) and blood platelets (thrombocytopenia). Symptoms and signs of agranulocytosis include infections of the throat, the gastrointestinal tract, and skin with an overall feeling of illness and fever. Since platelets are important for the clotting of blood, thrombocytopenia may lead to problems with excessive bleeding. Hepatitis and death of liver cells (hepatic necrosis) have rarely been associated with methimazole.

What is the dosage for methimazole?

The initial adult dose of methimazole is:

  • 15 mg/day for mild hyperthyroidism
  • 30-40 mg/day for moderately severe hyperthyroidism
  • 60 mg/day for severe hyperthyroidism

The daily dose is divided into three doses administered every 8 hours.

The maintenance dose is 5-15 mg/day.

The usual initial children's dose is 0.4 mg/kg given in 3 divided doses administered 8 hours apart, and the maintenance dose is half the initial dose.


Hyperthyroidism: Symptoms, Treatment, Medication See Slideshow

Which drugs interact with methimazole?

Warfarin works by reducing the activity of vitamin K and therefore the formation of vitamin K dependent clotting factors. Methimazole may increase the activity of warfarin by further reducing the activity of vitamin K in the body.

Hyperthyroidism increases elimination of beta blockers. Correcting hyperthyroidism will cause less elimination of beta blockers. Therefore, the dose of beta blockers may require reduction when hyperthyroidism is corrected. A similar reaction may also occur with digoxin (Lanoxin) and theophylline (Respbid, Slo-Bid, Theo-24, Theolair).

Pregnancy and breastfeeding

Methimazole is excreted in breast milk and may potentially cause harm to the infant.

What else should I know about methimazole?

What preparations of methimazole are available?

Tablet: 5, 10 mg

How should I keep methimazole stored?

Methimazole should be stored at room temperature, 15 C and 30 C (59 F and 86 F).


Methimazole (Tapazole) is a medication used to treat hyperthyroidism. Grave's disease is the most common cause of hyperthyroidism. Review side effects, drug interactions, dosage, and pregnancy safety information prior to taking methimazole.

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See more info: methimazole on RxList
Medically Reviewed on 5/10/2023
Medically reviewed by Eni Williams; PharmD., Ph.D. REFERENCE:

FDA Prescribing Information