Conversion of T4 to T3 is inhibited by all except
**Core Concept**
The conversion of T4 to T3 is a critical step in thyroid hormone metabolism, where thyroxine (T4) is converted to triiodothyronine (T3) by the enzyme 5'-deiodinase. This process is regulated by various factors, including thyroid hormone levels, thyroid-stimulating hormone (TSH), and other hormones.
**Why the Correct Answer is Right**
The conversion of T4 to T3 is inhibited by substances that increase T4 levels or block the action of 5'-deiodinase. This includes substances such as **propylthiouracil** (PTU), **methimazole**, and **amiodarone**, which are used to treat hyperthyroidism. PTU and methimazole inhibit the synthesis of thyroid hormones, while amiodarone inhibits the peripheral conversion of T4 to T3.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided, so we cannot evaluate its accuracy.
* **Option B:** This option is not provided, so we cannot evaluate its accuracy.
* **Option C:** This option is not provided, so we cannot evaluate its accuracy.
**Clinical Pearl / High-Yield Fact**
It's essential to note that the conversion of T4 to T3 is not the only pathway for thyroid hormone production. The thyroid gland also produces T3 through the action of thyroid-stimulating hormone (TSH) on the thyroid gland.
**Correct Answer:** D.