Which of the following drugs is an antithyroid medication?

Choice A rationale Propylthiouracil (PTU) is an antithyroid drug that works by inhibiting the enzyme thyroid peroxidase, which is responsible for the organification and coupling steps of thyroid hormone synthesis. This action reduces the production of new thyroid hormones, making it effective in treating hyperthyroidism. Choice B rationale Levothyroxine is a synthetic thyroid hormone used to treat hypothyroidism. It does not inhibit thyroid hormone production and is not an antithyroid medication; rather, it replaces deficient thyroid hormone, acting in direct opposition to antithyroid drugs. Choice C rationale Radioactive iodine is an antithyroid medication used to treat hyperthyroidism. When ingested, it is selectively absorbed by the thyroid gland, where its radiation destroys a portion of the thyroid tissue, thereby reducing the production of thyroid hormones. Choice D rationale Methimazole is a thionamide antithyroid drug that inhibits thyroid hormone synthesis by blocking the oxidation of iodide and the coupling of iodotyrosine residues, similar to PTU. It is a common and effective treatment for hyperthyroidism.