Which of the following drugs inhibit 5'– deiodinase?: (A) Propylthiouracil, (B) Methimazole, (C) Lugol's iodine, (D) Radioactive iodine
Answer is A. For the peripheral conversion of T1 to T3, the enzyme needed is 5'-deiodinase. It is inhibited by

Propylthiouracil
Propanolol
Amiodarone.