Which substance is specifically taken up by the thyroid gland?

• A. Potassium
• B. Iodine
• C. Catecholamines
• D. Water

Answer: (B)

Iodine is the substance the thyroid gland concentrates because it is essential for making thyroid hormones. The thyroid uses a sodium-iodide symporter to actively transport iodide from the blood into the thyroid follicular cells, concentrating it in the gland. Inside the follicle, iodide is oxidized and incorporated into tyrosine residues on thyroglobulin, forming mono- and di-iodotyrosines, which then couple to generate the hormones T3 and T4. This uptake and organification of iodine is unique to the thyroid, which is why iodine stands out as the substance specifically taken up by this gland.

Potassium, catecholamines, and water are not selectively accumulated by the thyroid in the same way or for the purpose of thyroid hormone synthesis. Potassium is a common intracellular ion, catecholamines are produced mainly in nerve and adrenal tissues, and water is distributed throughout the body.