Fastest acting antithyroid drug is?
**Core Concept**
The question is testing the knowledge of antithyroid drugs, specifically their onset of action. Antithyroid drugs are medications used to treat hyperthyroidism by inhibiting the production of thyroid hormones. The fastest acting antithyroid drug is crucial in managing thyroid emergencies, such as thyroid storm.
**Why the Correct Answer is Right**
The correct answer is **Propylthiouracil (PTU)**. PTU is a prodrug that is rapidly converted to its active metabolites, which inhibit the synthesis of thyroid hormones. It has a rapid onset of action, typically within 24-48 hours, making it the fastest acting antithyroid drug. PTU also has an additional effect of inhibiting the peripheral conversion of T4 to T3, which is beneficial in managing hyperthyroidism.
**Why Each Wrong Option is Incorrect**
**Option A:** Methimazole is a thionamide that is used as an antithyroid drug, but it has a slower onset of action compared to PTU, typically taking 2-6 weeks to take effect.
**Option B:** Carbimazole is a prodrug that is converted to methimazole, which is then responsible for its antithyroid effects. However, its onset of action is similar to methimazole, taking several weeks to take effect.
**Option C:** Iodine is used as an antithyroid drug, but it has a slower onset of action compared to PTU, typically taking several days to take effect.
**Clinical Pearl / High-Yield Fact**
It's essential to note that PTU is often used in the initial management of thyroid storm due to its rapid onset of action and additional effect of inhibiting peripheral conversion of T4 to T3.
**Correct Answer:** C. Propylthiouracil.