Which nutrient is essential for the synthesis of thyroid hormones?

• A. Vitamin C
• B. Calcium
• C. Iron
• D. Iodine

Answer: (D)

Think about what thyroid hormones are built from. Iodine is the essential element that gets incorporated into thyroglobulin in the thyroid gland to form the hormones T3 and T4. The thyroid adds iodine to tyrosine residues and couples them to create the active hormones. Without enough iodine, the gland can’t synthesize these hormones properly, leading to low metabolic activity and often an enlarged thyroid as it tries to trap more iodine. That biological role is why iodine is indispensable for thyroid hormone production.

Vitamin C, calcium, and iron don’t participate in making thyroid hormones. They have important roles elsewhere in the body, but the direct synthetic step for T3 and T4 requires iodine.