Question
Carbimazole
Answer
Carbimazole is a thioamide antithyroid drug, a prodrug converted to methimazole.
MOA: carbimazole → converted to methimazole → inhibits thyroid peroxidase → blocks iodide oxidation, organification, and coupling of iodotyrosines → new thyroid hormone synthesis is blocked (existing stored hormone still releases, so clinical effect takes weeks to appear).
Uses: hyperthyroidism (Graves’ disease, toxic nodular goitre), pre-operative preparation before thyroidectomy, adjunct before/after radioactive iodine therapy.
Adverse effects: agranulocytosis (the most feared, idiosyncratic — patients counselled to report fever/sore throat immediately), rash, hepatotoxicity, teratogenicity (aplasia cutis — avoided in the first trimester of pregnancy, when propylthiouracil is preferred instead).