Methimazole
TAPAZOLE
What it isThionamide inhibiting thyroid peroxidase; hyperthyroidism.
Why this oneMore potent and once daily compared with propylthiouracil, with less hepatotoxicity, so it is first-line outside the first trimester.
What limits itAgranulocytosis presents as fever and sore throat — patients must be told to stop and seek a count. First-trimester embryopathy.
FDA-approved for
- hyperthyroidism (~1950)
FDA label
Read the full label on DailyMed ↗
| FDA dosing | Population | Start | Target | Max |
|---|---|---|---|---|
| Hyperthyroidism (mild) | adult | 15 mg/day divided TID (q8h) | 5-15 mg/day (maintenance) | |
| Hyperthyroidism (moderately severe) | adult | 30-40 mg/day divided TID (q8h) | 5-15 mg/day (maintenance) | |
| Hyperthyroidism (severe) | adult | 60 mg/day divided TID (q8h) | 5-15 mg/day (maintenance) | |
| Hyperthyroidism | pediatric | 0.4 mg/kg/day divided TID (q8h) | ~1/2 of initial dose (maintenance) |
Renal Undefined
Hepatic Undefined
Instructions
- Give total daily dose in 3 divided doses at ~8-hour intervals
Cautions
- Agranulocytosis, granulocytopenia, thrombocytopenia, aplastic anemia
- Hepatotoxicity/hepatitis
- ANCA-associated vasculitis
- Drug fever, lupus-like syndrome
- Insulin autoimmune syndrome (hypoglycemic coma)
- Embryopathy risk in pregnancy (esp. first trimester)
Adverse reactions
- Skin rash, urticaria, pruritus
- Nausea, vomiting, epigastric distress
- Arthralgia, myalgia, paresthesia
- Loss of taste, hair loss
- Headache, drowsiness
Pregnancy Can cross placenta and cause fetal goiter/cretinism and congenital malformations; consider alternative anti-thyroid agent esp. in first trimester
Lactation Undefined
NO BOXED WARNINGS
Principal riskAgranulocytosis; first-trimester embryopathy
Cost, est. cash$5–$150/month
Generic entry2000
Classes and tags
Clinical profile
Legacy pregnancy categoryUnknown / historical label unavailable
Defining liabilityAgranulocytosis, hepatotoxicity and fetal toxicity require prompt response to fever or sore throat.
Mechanism of action: Methimazole ballicule
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