Which of the following drugs inhibit 5′-deiodinase?
**Core Concept**
The 5'-deiodinase enzyme plays a crucial role in the activation of thyroid hormones, specifically converting T4 to T3. Inhibiting this enzyme can lead to decreased levels of active thyroid hormones, affecting thyroid function.
**Why the Correct Answer is Right**
Propylthiouracil (PTU) is a medication used to treat hyperthyroidism and thyroid storm. It works by inhibiting the enzyme 5'-deiodinase, which is responsible for converting T4 to T3, thereby reducing the production of active thyroid hormones. PTU also blocks the synthesis of thyroid hormones by inhibiting the enzyme thyroid peroxidase (TPO). This dual mechanism of action makes PTU an effective treatment for hyperthyroidism.
**Why Each Wrong Option is Incorrect**
* **Option A:** Methimazole primarily inhibits thyroid peroxidase (TPO), not 5'-deiodinase.
* **Option B:** Iodine is used to suppress thyroid hormone synthesis by inhibiting the uptake of iodine by the thyroid gland.
* **Option D:** Carbimazole is a prodrug that is converted to methimazole, which primarily inhibits TPO.
**Clinical Pearl / High-Yield Fact**
In the treatment of hyperthyroidism, PTU is preferred over methimazole in the first trimester of pregnancy due to its lower risk of teratogenicity.
**Correct Answer:** B. Iodine is used to suppress thyroid hormone synthesis by inhibiting the uptake of iodine by the thyroid gland.