Methimazole

Methimazole is a thionamide antithyroid agent that inhibits the synthesis of thyroid hormones. It was first introduced as an antithyroid agent in 1949 and is now commonly used in the management of hyperthyroidism, particularly in those for whom more aggressive options such as surgery or radioactive iodine therapy are inappropriate. On a weight basis, methimazole is 10 times more potent than the other major antithyroid thionamide used in North America, propylthiouracil, and is the active metabolite of the pro-drug carbimazole, which is an antithyroid medication used in the United Kingdom and parts of the former British Commonwealth. Traditionally, methimazole has been preferentially used over propylthiouracil due to the risk of fulminant hepatotoxicity carried by the latter, with propylthiouracil being preferred in pregnancy due to a perceived lower risk of teratogenic effects. Despite documented teratogenic effects in its published labels, the true teratogenicity of methimazole appears to be unclear and its place in therapy may change in the future.