see also:

• hyperthyroidism
• propylthiouracil

• carbimazole is the first line thionamide in almost all patients with hyperthyroidism needing Rx with an antithyroid medication as it results in a more rapid improvement in thyroid hormone levels, has less hepatotoxicity, and can be given once daily due to its longer half life

• starting dose of carbimazole is 10–30 mg/day in 2–3 divided doses depending on severity of thyrotoxicosis, although larger doses may be used in severe disease
• Four weeks following initiation of therapy, clinical review with repeat thyroid function tests should be undertaken to avoid hypothyroidism
• Antithyroid drug therapy is tapered to a maintenance dose (usually carbimazole 2.5–10 mg) and ceased after 12–18 months of therapy

• relatively common effects:
  • Gastrointestinal effects (nausea, vomiting, gastric discomfort)
  • Rash (urticarial or macular)
  • arthralgia or fever - may be a sign of more serious effects so generally cease Rx
  • transient mild neutropenia
• agranulocytosis (neutrophil count <0.5×109/L) is a rare but life-threatening complication of both antithyroid drugs with an incidence of 0.2–0.5% and due to cross reactivity between the two antithyroid medications, agranulocytosis with one drug is an absolute contraindication to trialling the other
• cholestatic hepatitis
• Polyarthritis
• ANCA-positive vasculitis
• use in pregnancy
  • carbimazole during pregnancy has been associated with birth defects, including aplasia cutis and 'carbimazole embryopathy', characterised by choanal atresia or oesophageal atresia.
  • it is usually recommended that propylthiouracil be used in the first trimester and then changed to carbimazole in the second trimester ²⁾
• other effects see documentation