Which of the following antithyroid drugs acts primarily by inhibiting thyroid peroxidase and blocking iodine incorporation into thyroid hormones?

Explanation

## Mechanism of PTU **Key Point:** Propylthiouracil (PTU) is a thionamide antithyroid drug that inhibits thyroid peroxidase (TPO), the enzyme responsible for iodination and coupling of tyrosine residues to form T3 and T4. It also has a secondary benefit of inhibiting peripheral conversion of T4 to T3 via 5'-deiodinase. ## Comparison of Antithyroid Drugs | Drug | Primary Mechanism | Secondary Action | Onset | | --- | --- | --- | --- | | PTU | TPO inhibition (iodine incorporation) | Inhibits T4→T3 conversion | 1–2 weeks | | Methimazole | TPO inhibition (iodine incorporation) | None | 1–2 weeks | | Iodine (Lugol's) | Inhibits hormone release | Decreases gland vascularity | 12–48 hours | | Propranolol | β-blockade | None (symptomatic only) | Minutes | | Sodium perchlorate | Blocks iodine uptake | — | Days | **High-Yield:** PTU is preferred in pregnancy (first trimester) and thyroid storm because of its dual action and lower placental transfer compared to methimazole. Methimazole is preferred for maintenance therapy due to once-daily dosing and lower hepatotoxicity risk. **Clinical Pearl:** PTU carries a black box warning for hepatotoxicity (fulminant hepatic failure) and agranulocytosis; it is reserved for special situations (pregnancy, allergy to methimazole, thyroid storm).