A 21 year old student is found to have hyperthyroidism. She is counselled on treatment options including radioactive iodine and antithyroid medications. Carbimazole acts on which part of the thyroid hormone synthesis pathway?
**Core Concept**
Carbimazole is a prodrug that is converted to its active metabolite, methimazole, which inhibits the synthesis of thyroid hormones. This occurs through the inhibition of an enzyme responsible for the coupling of iodotyrosines to form triiodothyronine (T3) and thyroxine (T4).
**Why the Correct Answer is Right**
Methimazole acts on the thyroid peroxidase enzyme (TPO), which is crucial for the oxidation of iodide and the subsequent coupling of iodotyrosines to form T3 and T4. This step is essential for the final synthesis of thyroid hormones. By inhibiting TPO, methimazole effectively reduces the production of thyroid hormones, leading to a decrease in their circulating levels.
**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because carbimazole does not act on the sodium-iodide symporter (NIS), which is responsible for the uptake of iodide into the thyroid gland.
**Option B:** This is incorrect because carbimazole does not inhibit the deiodinases, which are enzymes responsible for the conversion of T4 to T3 and reverse T3.
**Option C:** This is incorrect because carbimazole does not act on the thyroid-stimulating hormone (TSH) receptor, which is responsible for the regulation of thyroid hormone synthesis by the pituitary gland.
**Clinical Pearl / High-Yield Fact**
It is essential to note that carbimazole can cause agranulocytosis, a potentially life-threatening condition characterized by a severe decrease in white blood cells. Regular monitoring of blood counts is necessary when using this medication.
**Correct Answer: C. thyroid peroxidase enzyme (TPO)**