Mechanism of action of propylthiouracil in hyperthyroidism is:
**Core Concept**
Propylthiouracil is a medication used to treat hyperthyroidism, specifically in cases of Graves' disease or thyroiditis. Its mechanism of action involves the inhibition of thyroid hormone synthesis and release.
**Why the Correct Answer is Right**
Propylthiouracil works by inhibiting the enzyme thyroid peroxidase (TPO), which is essential for the synthesis of thyroid hormones (T3 and T4). By blocking TPO, propylthiouracil reduces the production of these hormones, thereby alleviating symptoms of hyperthyroidism. Additionally, propylthiouracil also blocks the peripheral conversion of T4 to T3, further reducing the overall thyroid hormone activity.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because propylthiouracil does not directly affect the release of pre-formed thyroid hormones.
**Option B:** This option is incorrect because propylthiouracil does not directly inhibit the uptake of iodine by the thyroid gland.
**Option C:** This option is incorrect because propylthiouracil does not directly affect the secretion of thyroid-stimulating hormone (TSH) from the pituitary gland.
**Clinical Pearl / High-Yield Fact**
Propylthiouracil is often preferred over other antithyroid medications like methimazole in patients with liver disease or during pregnancy, as it has a lower risk of hepatotoxicity and teratogenicity.
**Correct Answer: C. Inhibition of thyroid peroxidase (TPO) enzyme.**