Which of the following inhibits peripheral conversion of Thyroxin to Triiodothyronine?

Ans. d (Propylthiouracil) (Ref. KDT, Pharmacology, 6th ed.,250)Propylthiouracil also inhibits peripheral conversion of T4 to T3 by D1 type of 5'D1, but not by D2 type. This may partly contribute to its effects. Methimazole and carbimazole do not have this action and may even antagonize that of propylthiouracil.Differences between propylthiouracil and carbimazole PropylthiouracilCarbimazole1.Dose to dose less potentAbout 5 x more potent2.Highly plasma protein boundLess bound3.Less transferred across placenta and in milkLarger amounts cross to foetus and in milk4.Plasma tl/2=l-2 hours6--10 hours5.Single dose acts for 4-8 hours12-24 hours6.No active metaboliteProduces active metabolite-methimazole7.Multiple (2-3) daily doses neededMostly single daily dose8.Inhibits peripheral conversion of T, to T3Does not inhibit T, to T3 conversion Antithyroid DrugsFeatures1. Carbimazole, methimazole, prophlthiouracilThyroid peroxidase inhibitors2. ProphythiouracilDrug of choice in pregnancy3. Thyroid constipating AgentLugol's Iodine, Potassium iodide4. Faster acting antithyroid drugLugol's Iodine, Potassium iodide5. Drug causing destruction of thyroid gland131I6. Drug inhibiting peripheral conversion of T4 and T3Propranolol, Prophylthiouracil, Lithium, Amiodoraone