Carbimazole is a prodrug for methimazole used to treat hyperthyroidism and thyrotoxicosis during pregnancy and breast breastfeeding. Carbimazole is rapidly metabolized to methimazole, which is responsible for the antithyroid activity. Carbimazole is an antithyroid agent that decreases the uptake and concentration of inorganic iodine by the thyroid, it also reduces the formation of di-iodotyrosine and thyroxine. Once converted to its active form of methimazole, it 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. Doses of carbimazole of 30 mg daily or 50 mg weekly have not adversely affected the few breastfed infants studied and no cases of thyroid function alteration have been reported among infants exposed to methimazole via breastmilk. Carbimazole is not approved for marketing in the United States by the U.S. Food and Drug Administration but is available in other countries.