METHIMAZOLE

METHIMAZOLE (meth-im'a-zole) Tapazole Classifications: hormone; antithyroid agent; Therapeutic:antithyroid Prototype: Propylthiouracil Pregnancy Category: D

Availability

5 mg, 10 mg, 15 mg, 20 mg tablets

Action

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.

Therapeutic Effect

Corrects hyperthyroidism by inhibiting synthesis of the thyroid hormone.

Uses

Hyperthyroidism and prior to surgery or radiotherapy of the thyroid; may be used cautiously to treat hyperthyroidism in pregnancy.

Contraindications

Pregnancy (category D).

Cautious Use

Other drugs known to cause agranulocytosis; bone marrow suppression; older adults; hepatic disease.

Route & Dosage

Hyperthyroidism Adult: PO 5–15 mg q8h Child: PO 0.2–0.4 mg/kg/d divided q8h

Administration

Oral

• 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, pancytopenia, and aplastic anemia. Musculoskeletal: Arthralgia. CNS: Peripheral neuropathy, drowsiness, neuritis, paresthesias, vertigo. Skin: Rash, alopecia, skin hyperpigmentation, urticaria, and pruritus. Urogenital: Nephrotic syndrome.

Interactions

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.

Pharmacokinetics

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% in urine within 24 h. Half-Life: 5–13 h.

Nursing Implications

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.

---

Common adverse effects in italic, life-threatening effects underlined; generic names in bold; classifications in SMALL CAPS; Canadian drug name; Prototype drug