What is the prohormone of thyroglobulin?

• A. Thyroglobulin
• B. Tyrosine
• C. MIT
• D. DIT

Answer: (A)

Thyroglobulin acts as the prohormone. It is a large glycoprotein produced by thyroid follicular cells and stored in the colloid. Within thyroglobulin, tyrosine residues are iodinated to form MIT and DIT; these iodinated residues then couple (MIT + DIT forms T3, DIT + DIT forms T4) to build the thyroid hormones on the thyroglobulin scaffold. When needed, thyroglobulin is proteolyzed to release the active hormones, T3 and T4. So the precursor to the thyroid hormones is thyroglobulin itself, with MIT and DIT serving as intermediates inside it.