Which thyroid cells produce T3 and T4?

• A. Follicular cells
• B. Parafollicular cells
• C. Colloid cells
• D. C cells

Answer: (A)

Thyroid hormones T3 and T4 are produced by the follicular cells that line the thyroid follicles. These cells synthesize thyroglobulin and actively take up iodide from the blood, delivering it to the colloid. In the colloid, thyroid peroxidase enables iodination of tyrosine residues on thyroglobulin and couples them to form MIT and DIT; coupling of two DIT yields T4, while a DIT and an MIT yield T3. The hormones remain bound to thyroglobulin in the colloid until stimulated by TSH, at which point proteolysis releases free T3 and T4 into circulation. Parafollicular (C) cells produce calcitonin, not thyroid hormones, and colloid is the storage substance inside follicles, not a cell.