Drug Breakdown: Carbimazole

Dr Sharon Rees @reesprescribe

Day 1: Sulphur-containing compounds were found to be thyroid-toxic in the 1940s, which led to propylthiouracil use. A second thiouracil analogue ‘methimazole’ was found and its prodrug #carbimazole was licensed in 1953, as it was believed to be less toxic, which was not the case.

Dr Sharon Rees @reesprescribe

Day 2: #carbimazole is used to treat hyperthyroidism for all ages. The adult starting dose is 20–60 mg daily (in divided doses) and the child dose is dependent on age/weight; all regimens are based on time to euthyroid, then reduce to the maintenance dose. #carbimazole is also used for the ‘blocking-replacement’ regimen in combination for 6–18 months with levothyroxine to prevent hypothyroidism.

Dr Sharon Rees @reesprescribe

Day 3: Oral absorption is unaffected by food; metabolite cmax 1–2 hours. #carbimazole has a low volume of distribution as it concentrates in the thyroid gland. Crosses the placenta and presents in breast milk. 10% entero-hepatic recycling and contraindicated if severe hepatic impairment. T½ 5–6 hrs (prolonged in hyperthyroidism). Renal excretion is approximately 90% as the drug is mostly unchanged.