Question

The mechanism of action of methimazole used to treat hyperthyroidism is

• A. It blocks the release of stored thyroid hormone from the gland.
• B. It replaces deficient thyroid hormone in the bloodstream.
• C. It inhibits thyroid peroxidase, reducing synthesis of thyroid hormones.
• D. It destroys thyroid tissue through radiation.

Hint:

Methimazole treats hyperthyroidism by inhibiting thyroid peroxidase and reducing new thyroid hormone synthesis.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Hyperthyroidism involves the overproduction of thyroid hormones (\(T_3\) and \(T_4\)). Thioamide drugs are used to slow down this production.
Step 2: Detailed Explanation:
1. Methimazole: It is an antithyroid drug. It works by inhibiting the enzyme thyroid peroxidase (TPO).
2. Mechanism: This enzyme is responsible for the iodination of tyrosine residues on thyroglobulin and the subsequent coupling of these residues to form \(T_3\) and \(T_4\). By blocking TPO, the actual synthesis of new hormones is stopped.
3. Others: Option (A) describes Iodides (high doses). Option (D) describes Radioactive Iodine (\(I^{131}\)). Option (B) describes Levothyroxine (used for hypothyroidism).
Step 3: Final Answer:
Methimazole's primary mechanism is the inhibition of hormone synthesis via the blockade of the thyroid peroxidase enzyme.