**Core Concept**
Thyroid hormone synthesis and secretion are regulated by various medications, which can be used to manage hyperthyroidism. Antithyroid drugs primarily target the thyroid gland to reduce thyroid hormone production.

**Why the Correct Answer is Right**
Antithyroid drugs work by either inhibiting the synthesis of thyroid hormones or blocking their release. The most commonly used antithyroid drugs are methimazole (MMI), propylthiouracil (PTU), and carbimazole. MMI and carbimazole inhibit the enzyme thyroid peroxidase (TPO), which is crucial for the synthesis of thyroid hormones. PTU, on the other hand, inhibits TPO and also blocks the peripheral conversion of T4 to T3.

**Why Each Wrong Option is Incorrect**
**Option A:** Methimazole is an antithyroid drug that inhibits thyroid hormone synthesis.
**Option B:** Propylthiouracil is another antithyroid drug that blocks thyroid hormone synthesis and peripheral conversion.
**Option C:** Radioactive iodine is a treatment for hyperthyroidism that destroys thyroid tissue, but it is not classified as an antithyroid drug.

**Clinical Pearl / High-Yield Fact**
When prescribing antithyroid drugs, it is essential to monitor liver function in patients taking PTU due to the risk of hepatotoxicity. Additionally, MMI and carbimazole can cause agranulocytosis, a potentially life-threatening condition.

**Correct Answer:** C. Radioactive iodine is a treatment for hyperthyroidism that destroys thyroid tissue, but it is not classified as an antithyroid drug.