Iodine deficiency would most directly affect the production of which hormone?

• A. Melatonin
• B. Glucagon
• C. Thyroxine
• D. Parathyroid hormone

Answer: (C)

Iodine is essential for making thyroid hormones, so a deficiency directly reduces the thyroid’s ability to synthesize thyroxine (T4) and, after conversion, the active hormone T3. Thyroxine is the major hormone produced by the thyroid, and without enough iodine, its production drops, leading to low thyroid hormone levels and often a rise in TSH with possible goiter. The other hormones listed don’t rely on iodine for their synthesis: melatonin comes from serotonin in the pineal gland, glucagon from pancreatic alpha cells, and parathyroid hormone from the parathyroids. So the hormone most directly affected by iodine deficiency is thyroxine.