Overview
Carbimazole is an antithyroid agent used primarily in the treatment of hyperthyroidism, particularly Graves’ disease. It acts to reduce the production of thyroid hormones, helping to restore a normal thyroid state (euthyroidism).

Mechanism of Action
It is a pro-drug that undergoes rapid conversion to methimazole in the body. Methimazole inhibits the enzyme thyroid peroxidase, thereby preventing the incorporation of iodine into thyroglobulin and blocking the synthesis of thyroid hormones (T3 and T4).

Dosage and Administration
The starting dose is typically 15 to 40 mg daily, given in divided doses or as a single daily dose, depending on the severity of hyperthyroidism. Once the patient becomes euthyroid, the dose is gradually reduced to a maintenance level, usually 5 to 15 mg daily.

Side Effects
Common side effects include skin rashes, pruritus, and mild gastrointestinal upset. A rare but life-threatening complication is agranulocytosis (a severe drop in white blood cells), which presents as a sore throat or fever and requires immediate cessation of the drug.

Contraindications
It is contraindicated in patients with a history of severe adverse reactions to carbimazole or methimazole, such as agranulocytosis or severe hepatic impairment. Caution is required in patients with mild to moderate liver disease.

Pregnancy and Lactation
Carbimazole can cross the placenta and may cause fetal goiter or hypothyroidism. If antithyroid therapy is necessary during pregnancy, propylthiouracil is generally preferred in the first trimester, though carbimazole may be used cautiously in the second and third trimesters. It is excreted in breast milk.