methimazole

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.

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:

• rash,
• itching,
• hives,
• abnormal hair loss, and
• skin pigmentation.

Other common side effects are:

• swelling,
• nausea,
• vomiting,
• heartburn,
• loss of taste,
• joint or muscle aches,
• numbness and
• headache.

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.

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.

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).

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

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).