Which statement accurately describes the roles of selenium and iodine in thyroid function?

• A. Selenium is a cofactor for thyroid hormone metabolism; iodine is essential for thyroid hormone synthesis.
• B. Selenium is not involved in thyroid function; iodine is a cofactor for thyroid hormone metabolism.
• C. Iodine is a cofactor for thyroid hormone metabolism; selenium is required for thyroid hormone storage.
• D. Neither selenium nor iodine play a role in thyroid function.

Answer: (A)

Thyroid hormone production depends on iodine to be incorporated into thyroglobulin, forming the thyroid hormones T3 and T4. Selenium plays a crucial role after synthesis as a cofactor for the enzymes that metabolize thyroid hormones, notably the deiodinases that convert the less active T4 into the active T3 and regulate hormone availability. Selenium is also a component of selenoproteins like glutathione peroxidases that protect thyroid cells from oxidative damage produced during hormone synthesis. This combination explains why selenium supports thyroid hormone metabolism and iodine is essential for making the hormones themselves. The statement that selenium serves as a cofactor for hormone metabolism and that iodine is needed for hormone synthesis accurately reflects their roles. The other ideas—selenium not being involved, iodine acting as a metabolism cofactor, or selenium being tied to storage—do not fit what these nutrients actually do in thyroid function.