Classifications: hormone; antithyroid agent; Therapeutic:antithyroid
Pregnancy Category: D
5 mg, 10 mg, 15 mg, 20 mg tablets
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 (category D).
Other drugs known to cause agranulocytosis; bone marrow suppression; older adults; hepatic disease.
Route & Dosage
Adult: PO 515 mg q8h
Child: PO 0.20.4 mg/kg/d divided q8h
- Give at same time each day relative to meals.
- Store at 15°30° C (59°86° F) in light-resistant container.
Adverse Effects (≥1%)GI: hepatotoxicity (rare). Endocrine:
Hypothyroidism. Hematologic: Leukopenia,
agranulocytosis, granulocytopenia, thrombocytopenia
, and aplastic anemia
. Musculoskeletal: Arthralgia
, drowsiness, neuritis
, paresthesias, vertigo. Skin:
, skin hyperpigmentation, urticaria, and pruritus. Urogenital:
Can reduce anticoagulant effects of warfarin;
may increase serum
levels of digoxin;
may alter theophylline
levels; may need to decrease dose of beta-blockers
Readily absorbed from GI tract. Onset:
3040 min. Peak:
1 h. Duration:
24 h. Distribution:
Crosses placenta; distributed into breast milk. Elimination:
12% in urine within 24 h. Half-Life:
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
- 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.