Source: Medicines & Healthcare Products Regulatory Agency (GB) Revision Year: 2019 Publisher: Amdipharm UK Limited, Capital House, 85 King William Street, London EC4N 7BL, UK
Pharmacotherapeutic group: Sulfur-containing imidazole derivatives
ATC Code: H03BB
Carbimazole, a thionamide, is a pro-drug which undergoes rapid and virtually complete metabolism to the active metabolite, thiamazole, also known as methimazole. The method of action is believed to be inhibition of the organification of iodide and the coupling of iodothyronine residues which in turn suppress the synthesis of thyroid hormones.
Carbimazole is rapidly metabolised to thiamazole. After oral ingestion, peak plasma concentrations of thiamazole, the active moiety, occur at 1 to 2 hours
The total volume of distribution of thiamazole is 0.5 1/kg. Thiamazole is concentrated in the thyroid gland. This intrathyroidal concentration of thiamazole has the effect of prolonging its activity. However, thiamazole has a shorter half-life in hyperthyroid patients than in normal controls and so more frequent initial doses are required while the hyperthyroidism is active.
Thiamazole is moderately bound to plasma proteins.
Carbimazole has a half-life of 5.3 to 5.4 hours. It is possible that the plasma half-life may also be prolonged by renal or hepatic disease. See section 4.2. Thiamazole crosses the placenta and appears in breast milk. The plasma milk ratio approaches unity.
Over 90% of orally administered carbimazole is excreted in the urine as thiamazole or its metabolites. The remainder appears in faeces. There is 10% enterohepatic circulation.
There are no preclinical data of relevance to the prescriber which are additional to that already included in other sections of the Summary of Product Characteristics.
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