What is the role of iodine in thyroid hormone production?

• A. Iodine reduces TSH production
• B. Iodine is only used in parathyroid function
• C. Iodine is essential for synthesis of thyroid hormones
• D. Iodine deficiency has no effect on thyroid function

Answer: (C)

Iodine is essential for making thyroid hormones. The thyroid concentrates iodide from the blood and uses thyroid peroxidase to oxidize and attach it to tyrosine residues on thyroglobulin, forming MIT and DIT. These iodinated units then couple to produce the hormones T3 and T4, which are stored in thyroglobulin and released when needed. The availability of iodine directly limits how much hormone can be synthesized, so adequate iodine supports normal T3 and T4 production and proper feedback to regulate TSH. If iodine is deficient, hormone production drops, TSH rises, and the thyroid may enlarge (goiter) in an attempt to trap more iodine. Iodine doesn’t directly reduce TSH, nor is it only used in parathyroid function, and iodine deficiency clearly affects thyroid function, making the statement that it has no effect incorrect.