Methimazole
Brand names: Tapazole
Class: 🦋 Thyroid & Antithyroid
On board day, methimazole is first-line for Graves disease and toxic adenoma in non-pregnant adults. It inhibits thyroid peroxidase, reducing T4 and T3 synthesis. Used either as definitive therapy in mild Graves or as preparation for radioactive iodine ablation or thyroidectomy. The pregnancy rule is the high-yield AANP point: methimazole causes embryopathy in the first trimester, so switch to propylthiouracil for the first trimester and back to methimazole for the second and third (PTU has more hepatotoxicity beyond early pregnancy). Both agents carry agranulocytosis and hepatotoxicity warnings — counsel any patient who develops fever and sore throat to stop the drug and check a CBC immediately.
✅ Indications
Hyperthyroidism (Graves, toxic adenoma), preparation for thyroidectomy or RAI, 2nd-3rd trimester pregnancy.
⚙️ Mechanism of Action
Thioamide — inhibits thyroid peroxidase → ↓ T3/T4 synthesis.
📏 Dosing
5–40 mg PO daily (divided if >15 mg).
🚫 Contraindications
1st trimester pregnancy (switch to PTU), severe hepatic disease, history of agranulocytosis on thioamide.
⚠️ Adverse Effects
Rash, nausea, AGRANULOCYTOSIS (rare, severe), hepatotoxicity, cholestatic jaundice.
🔬 Monitoring
TSH, T4, CBC (baseline + with fever/sore throat), LFTs.
💎 Board Pearls
- 🔄 1st trimester = PTU. 2nd/3rd trimester = methimazole (PTU hepatotoxicity risk).
- 🔥 Agranulocytosis warning: fever + sore throat → STOP + CBC.
- 💊 First-line hyperthyroidism drug outside 1st-trimester pregnancy (1 daily vs PTU TID).
Practice Questions
A 42-year-old woman started on methimazole (Tapazole) 20 mg daily for Graves disease three weeks ago calls the clinic reporting a fever of 38.9°C, severe sore throat, and chills that began overnight. She has no cough or congestion. Which response is MOST appropriate?
Related Drugs in This Class
- Levothyroxine — Synthroid, Levoxyl, Tirosint
- Propylthiouracil — PTU
Sources
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