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Carbimazole is used to treat hyperthyroidism. Carbimazole is a pro-drug as after absorption it is converted to the active form, methimazole. Methimazole prevents the thyroid peroxidase enzyme from coupling and iodinating the tyrosine residues on thyroglobulin, hence reducing the production of the thyroid hormones T3 and T4 (thyroxine).
Therapy for hyperthyroidism generally starts at a high daily dose of 15–40 mg continued until the patient has normal thyroid function, and then reduced to a maintenance dose of 5–15 mg. Treatment is usually given for 12–18 months followed by a trial withdraw.
The onset of anti-thyroid effect is rapid but the onset of clinical effects on thyroid hormone levels in the blood is much slower. This is because the large store of pre-formed T3 and T4 in the thyroid gland and bound to thyroid binding globulin (99% bound) has to be depleted before any beneficial clinical effect occurs.
Carbimazole should be used judiciously in pregnancy as it crosses the placenta. It has (rarely) been associated with congenital defects, including aplasia cutis of the neonate but is not contra-indicated. However, it more predictably may cause fetal hypothyroidism so (in minimal doses) it can be used in order to control maternal hyperthyroidism. There are reported cases of goiter and choanal atresia in fetus. Furthermore, breast feeding is possible but only if lowest effective dose is used and neonatal development is closely monitored.
Whilst rashes and pruritus are common, these can often be treated with antihistamines without stopping the carbimazole. For those patients where sensitivity reactions can not be controlled, propylthiouracil may be used as an alternative; cross-sensitivity between these drugs is rare.
Its most serious rare side effect is bone marrow suppression causing neutropenia and agranulocytosis. This may occur at any stage during treatment and without warning; monitoring of white cell count is not useful. Patients are advised to immediately report symptoms of infection, such as sore throat or fever, so that a full blood count test may be arranged. If this confirms a low neutrophil count, discontinuation of the drug leads to recovery. However failure to report suggestive symptoms or delays in considering the possibility of immunosuppression and its testing, can lead to fatalities.
- Neomercazole 
- British National Formulary 45 March 2003