Which of the following pharmacologic agents can be used in the treatment of thyrotoxicosis to block the production of thyroid hormone?

a. Propylthiouracil

c. Methimazole

d. Carbimazole

e. Iodine 

A number of substances interfere with normal production of thyroid hormone by blocking one of the relevant enzymatic steps. Iodine was the first effective drug to be used in the treatment of thyrotoxicosis. It can block the organification and coupling steps in thyroid hormone synthesis as well as prevent the release of thyroid hormone. Furthermore, iodine in large doses probably inhibits the ability of TSH to stimulate cyclic AMP release at the follicular cell membrane. Commonly used antithyroid drugs are propylthiouracil (PTU), methimazole (Tapazole) and carbimazole. Carbimazole is widely used in Great Britain, only PTU and methimazole are commonly used in the United States. PTU interferes with the incorporation of iodine into the tyrosine residues of thyroglobulin, thus preventing oxidation of iodide to iodine. It also inhibits the peripheral conversion of T4 to T3. Although both PTU and methimazole are thionamides, methimazole does not have this peripheral effect, making PTU the preferred drug for patients with thyroid storm. About 3% of patients taking PTU demonstrate at least one side effect during the first 3 months of therapy whereas the prevalence with methimazole is about 7%. These range from minor skin rashes to agranulocytosis that can be irreversible. b-adrenergic antagonists have been used in the treatment of hyperthyrodism. Propranolol is the most widely used of these drugs. However, b-blockade does not alter thyroid function per se. Rather, its effect is to provide symptomatic relief of hyperthyroidism because of interference with the action of thyroid hormones at the cellular level.