A 62-year-old female with hypothyroidism takes levothyroxine (Synthroid®). The medication used for her hypothyroidism replaces a hormone normally produced by which cells?

Correct answer: C. Thyroid follicular cells

• A. INCORRECT. Chief cells (principal cells) are found in the parathyroid glands and produce parathyroid hormone (PTH), which regulates calcium and phosphate metabolism. They do not produce thyroid hormone.
• B. INCORRECT. Parafollicular cells (C cells) of the thyroid gland produce calcitonin, which lowers blood calcium levels. Levothyroxine replaces T4 (thyroxine), not calcitonin.
• C. CORRECT. Thyroid follicular cells (principal/follicular epithelial cells) are responsible for the synthesis and secretion of the thyroid hormones T3 (triiodothyronine) and T4 (thyroxine). Levothyroxine is a synthetic form of T4. In hypothyroidism, these cells fail to produce adequate hormone.
• D. INCORRECT. Glomerulus parietal cells are not an established anatomical/histological cell type in standard terminology. Parietal cells are found in the stomach fundus and produce hydrochloric acid and intrinsic factor — they have no relation to thyroid hormone production.