METHIMAZOLE
(meth-im'a-zole)
Tapazole
Classifications: hormones and synthetic substitutes; antithyroid agent
Prototype: Propylthiouracil
Pregnancy Category: D
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5 mg, 10 mg tablets
Thioamide with actions and uses similar to those of propylthiouracil but 10 times as potent. Actions are less consistent,
but effects appear more promptly than with propylthiouracil. Inhibits synthesis of thyroid hormones as the drug accumulates
in the thyroid gland. Does not affect existing T3 or T4 levels.
Corrects hyperthyroidism by inhibiting synthesis of the thyroid hormone.
Hyperthyroidism and prior to surgery or radiotherapy of the thyroid; may be used cautiously to treat hyperthyroidism in pregnancy.
Pregnancy (category D), lactation.
Other drugs known to cause agranulocytosis.
Hyperthyroidism
Adult: PO 5–15 mg q8h
Child: PO 0.2–0.4 mg/kg/d divided q8h
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Oral
- Give at same time each day relative to meals.
- Store at 15°–30° C (59°–86° F) in light-resistant container.
GI:
hepatotoxicity (rare). Endocrine: Hypothyroidism. Hematologic:
Leukopenia, agranulocytosis, granulocytopenia, thrombocytopenia, pancytopenia, and aplastic anemia. Musculoskeletal: Arthralgia. CNS: Peripheral neuropathy, drowsiness, neuritis, paresthesias, vertigo. Skin: Rash, alopecia, skin hyperpigmentation, urticaria, and pruritus. Urogenital: Nephrotic syndrome.
Drug: Can reduce anticoagulant effects of warfarin; may increase serum levels of digoxin; may alter theophylline levels; may need to decrease dose of beta-blockers.
Absorption: Readily absorbed from GI tract. Onset: 30–40 min. Peak: 1 h. Duration: 2–4 h. Distribution: Crosses placenta; distributed into breast milk. Elimination: 12% excreted in urine within 24 h. Half-Life: 5–13 h.
Assessment & Drug Effects
- Lab tests: Periodic blood work, since agranulocytosis is a rare, but possible adverse effect.
- Closely monitor PT and INR in patients on oral anticoagulants. Anticoagulant activity may be potentiated.
Patient & Family Education
- Adhere to established dosage regimen (i.e., not to double, decrease, or omit doses and not to alter the interval between doses).
- Be aware that skin rash or swelling of cervical lymph nodes may indicate need to discontinue drug and change to another antithyroid
agent. Consult physician.
- Notify physician promptly if the following symptoms appear: Bruising, unexplained bleeding, sore throat, fever, jaundice.
- Drug-induced jaundice may persist up to 10 wk after withdrawal of drug.
- Methimazole does not induce hypothyroiditis.
- Do not breast feed while using this drug.
1%)
Canadian drug name; 
Prototype drug