Which medication acts by inhibiting the synthesis of thyroid hormones?

Methimazole is an antithyroid medication that specifically works by inhibiting the synthesis of thyroid hormones. It achieves this by blocking the enzyme thyroid peroxidase, which is crucial in the process of incorporating iodine into the tyrosine molecules that are part of the thyroglobulin, ultimately preventing the formation of thyroxine (T4) and triiodothyronine (T3). By doing so, it reduces the levels of these hormones in the bloodstream, making it an effective treatment for conditions like hyperthyroidism.

Other choices do not inhibit thyroid hormone synthesis. Levothyroxine is a synthetic form of the thyroid hormone T4 and is used for hormone replacement therapy rather than inhibition. Propranolol is a beta-blocker that can help manage symptoms of hyperthyroidism but does not directly affect hormone synthesis. Sodium iodide is used in the treatment of hyperthyroidism and can also be involved in the destruction of overactive thyroid tissue, but it does not inhibit hormone synthesis directly. Therefore, Methimazole is recognized for its specific role in inhibiting the synthesis of thyroid hormones.