Question

Antithroid drug inhibiting hormone synthesis is ________.

• A. propranolol
• B. methimazole
• C. levothyroxine
• D. iodothyronine

Hint:

Antithyroid mechanisms: - Synthesis Inhibitors: Methimazole, Propylthiouracil (PTU) $\implies$ target thyroid peroxidase. - PTU has an added benefit of inhibiting peripheral $T_4$ to $T_3$ conversion, making it useful in acute thyroid storms.

Answer 1

The Correct Option is B

Understanding the Concept: Thyroid hormone synthesis requires thyroid peroxidase (TPO), an enzyme that catalyzes both the oxidation of iodide ions and the iodination of tyrosine residues on thyroglobulin (organification), as well as the coupling of iodotyrosines ($MIT$ and $DIT$) to form $T_3$ and $T_4$. Antithroid drugs classified as Thionamides lower hormone production by inhibiting these enzymatic processes.

Step 1: Identify the thionamide derivative.
Methimazole is a potent thionamide that binds to and inhibits the thyroid peroxidase enzyme. This directly blocks the organification of iodine and the coupling reactions, preventing the synthesis of new thyroid hormones.

Step 2: Evaluate alternative options.
* Propranolol is a $\beta$-adrenergic receptor blocker used to manage symptomatic cardiovascular features of hyperthyroidism (thyrotoxicosis) and partially inhibit peripheral conversion of $T_4$ to $T_3$, but it does not inhibit hormone synthesis in the thyroid gland. * Levothyroxine is synthetic thyroid hormone ($T_4$) used to treat hypothyroidism.