Carbimazole is an antithyroid agent which depresses the formation of thyroid hormones. Its main action is the reduction of formation of iodotyrosines by inhibition on the iodination of thyroglobulin and reduction of the synthesis of tri-iodothyronine and thyroxine by inhibition of the coupling reaction between iodotyrosines. The release of any preformed thyroid hormones is not affected. Carbimazole does not reduce the uptake of iodide by the thyroid gland. It is considered that the clinical antithyroid activity of Carbimazole is attributable to its major metabolite Methimazole. Both Carbimazole and Methimazole possess some immunosuppressant activity.