This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pharmacology
Which of the following is true of propylthiouracil? 
Select and explain. 
A. It is used in the treatment of myxedema, B. It has a long half-life in the circulation, C. It increases proteolysis of thyroglobulin, D. It blocks thyroxine synthesis
D: It blocks thyroxine synthesis
Answer: D: It blocks thyroxine synthesis 
Explanation: Ans. is 'd' i.e., It blocks thyroxine synthesis o Propylthiouracil blocks thyroxine synthesis by inhibiting thyroid peroxidase, and preventing oxidation of iodide/ iodotyrosyl residues, there by Inhibit iodination of tyrosine residue in thyroglobulin. Inhibit coupling of MIT and DIT to form T3 & T4. o Propylthiouracil also inhibit the peripheral conversion of T4 to T3. About other options o It is a hypothyroid drug, cannot be used in myxedema (hypothyroidism). It has a sho VA - 1-2 hours. It does not increase proteolysis of thyroxine.