Carbimazole

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 peroxidase enzyme from coupling and iodinating the tyrosine residues on thyroglobulin, hence reducing the production of the thyroid hormones T3 and T4 (thyroxine).

Clinical use
Therapy for hyperthyroidism generally starts at a high dose of 15 - 40mg continued until the patient has normal thyroid function, and then reduced to a maintenance dose of 5 - 15mg. 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 has to be depleted before any beneficial clinical effect occurs.

Side Effects
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.

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. Patients are advised to immediately report symptoms of infection, especially sore throats, so that a full blood count test may be arranged. If this confirms a low neutrophil count then the drug must be discontinued immediately, allowing for usually a prompt recovery. However failure to report suggestive symptoms or delays in considering the possibility of immunosuppresion and its testing, can lead to fatalities.