Medical Author: Omudhome Ogbru, Pharm.D.
Medical Editor: Jay Marks, M.D.
GENERIC NAME: methimazole
BRAND NAME: Tapazole, Northyx
DRUG CLASS AND MECHANISM: Methimazole is used to treat an overactive
thyroid gland (hyperthyroidism).
Grave's disease is the most common cause of hyperthyroidism. It 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.
GENERIC AVAILABLE: Yes
PRESCRIPTION: Yes
PREPARATIONS: Tablet: 5, 10, and 15 mg
STORAGE: Methimazole should be stored at room temperature, 15-30 C
(59-86 F).
PRESCRIBED FOR: Methimazole is used for treating 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.
DOSING: The initial adult dose of methimazole is:
- 15 mg/day for mild
hyperthyroidism,
- 30-40 mg/day for moderately severe hyperthyroidism, and
- 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.
DRUG INTERACTIONS: Warfarin (Coumadin) 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.
PREGNANCY: There is evidence that methimazole may cause harm to the
fetus during pregnancy.
NURSING MOTHERS: Methimazole is excreted in
breast milk and may
potentially cause harm to the infant.
SIDE EFFECTS: 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.
Last Editorial Review: 12/17/2008
Report Problems to the Food and Drug Administration
You are encouraged to report negative side effects of prescription drugs to the FDA. Visit the FDA MedWatch website or call 1-800-FDA-1088.
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