Which hormone is produced entirely within the thyroid gland?

• A. T4
• B. T3
• C. Both T3 and T4
• D. None

Answer: (A)

The thyroid gland is responsible for making two main hormones by iodinating tyrosine residues in thyroglobulin within the follicular cells: T4 (thyroxine) and T3 (triiodothyronine). The important distinction is how these hormones become active in the body. The thyroid secretes T4 in large amounts, and peripheral tissues—like the liver and kidneys—remove an iodine from T4 to convert it into the more active T3. Because this conversion happens outside the thyroid, the hormone produced entirely within the thyroid gland is T4. T3 is also produced in the thyroid, but most of the circulating T3 results from peripheral conversion of T4, so it is not produced entirely within the thyroid. None of the other options fit the statement as accurately.