Four pea-sized glands in the thyroid gland that produce parathyroid hormone to regulate blood calcium and phosphate levels?

• A. Parathyroid glands
• B. Thyroid gland
• C. Thyroxine (T4)
• D. Aldosterone

Answer: (A)

Calcium homeostasis is controlled by parathyroid hormone released from four small glands located on the back surface of the thyroid. When blood calcium drops, these parathyroid glands secrete PTH, which raises calcium levels by stimulating bone resorption, increasing kidney calcium reabsorption, and promoting activation of vitamin D to boost intestinal calcium absorption. This description matches the parathyroid glands, making them the correct choice. The thyroid gland itself makes thyroid hormones that regulate metabolism (and calcitonin, which can lower calcium), while aldosterone is produced by the adrenal cortex and has a different role in electrolyte balance.