Which enzyme converts T4 to T3?

• A. iodothyronine 5'-deiodinase
• B. Thyroid Peroxidase
• C. Deiodinase Type 3
• D. Tyrosine Kinase

Answer: (A)

Converting T4 to the active T3 happens through outer-ring deiodination, where an iodine atom is removed from the outer ring of thyroxine. The enzyme that does this in body tissues is iodothyronine 5'-deiodinase. This enzyme’s action essentially activates thyroid hormone by generating T3 from T4, which is why it’s the correct choice.

Thyroid peroxidase, on the other hand, is involved in making thyroid hormones inside the thyroid gland itself—organifying iodine and coupling the iodotyrosines—not in converting T4 to T3. Deiodinase Type 3 performs the opposite reaction, inactivating thyroid hormones by converting T4 to reverse T3 or T3 to T2. Tyrosine kinase is a signaling enzyme and does not participate in thyroid hormone activation.