Which of the following is not true regarding carbimazole: September 2010
**Core Concept**
Carbimazole is a prodrug that is converted to its active metabolite, methimazole, which inhibits the synthesis of thyroid hormones by blocking the enzyme thyroid peroxidase (TPO). This action results in reduced production of thyroxine (T4) and triiodothyronine (T3).
**Why the Correct Answer is Right**
Carbimazole works by inhibiting the enzyme thyroid peroxidase, which is essential for the incorporation of iodine into tyrosine residues on the thyroglobulin molecule, forming monoiodotyrosine (MIT) and diiodotyrosine (DIT). These iodinated tyrosines are then coupled to form T3 and T4. By blocking this enzyme, carbimazole reduces the production of thyroid hormones.
**Why Each Wrong Option is Incorrect**
**Option A:** Carbimazole is used to treat hyperthyroidism. This is true, as carbimazole is used to reduce the production of thyroid hormones.
**Option B:** Carbimazole can cross the placenta. This is true, and it highlights the need for careful management of thyroid function in pregnant women taking carbimazole.
**Option C:** Carbimazole can cause agranulocytosis. This is true, and it is a rare but serious side effect of carbimazole.
**Clinical Pearl / High-Yield Fact**
Carbimazole is a prodrug that requires metabolic activation to its active metabolite, methimazole, which is responsible for its therapeutic and side effects.
**Correct Answer: A. This statement is true.**