Which enzyme catalyzes the iodination and coupling steps forming MIT/DIT and T3/T4 during thyroid hormone synthesis?

• A. Thyroid peroxidase
• B. Deiodinase
• C. TSH
• D. Thyroid hormone receptor

Answer: (A)

Thyroid peroxidase sits at the heart of the hormone-making steps that assemble MIT and DIT and then couple them to form T3 and T4. It operates in the colloid of the thyroid follicles, using hydrogen peroxide to oxidize iodide and attach it to tyrosine residues on thyroglobulin, creating the iodinated precursors MIT and DIT. Those same catalytic actions then enable the coupling reactions that join MIT and DIT to produce T3, and DIT with DIT to produce T4, all on the thyroglobulin scaffold. This is the enzyme responsible for both the iodination and the coupling steps in thyroid hormone synthesis.

Other components have different roles: deiodinase converts T4 to T3 in peripheral tissues; TSH mainly stimulates the gland’s activity and hormone release but is not the catalytic enzyme; the thyroid hormone receptor is the nuclear receptor that mediates the action of T3/T4 after they're made.