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