The regulation of calcium and phosphorus metabolism is accomplished by which gland?

• A. Thyroid
• B. Parathyroid
• C. Adrenal gland
• D. Pituitary

Answer: (B)

Calcium and phosphorus metabolism is governed primarily by parathyroid hormone from the parathyroid glands. This hormone acts to raise blood calcium when it falls by three main actions: triggering bone resorption to release calcium, increasing renal reabsorption of calcium in the distal tubules, and stimulating production of calcitriol (active vitamin D) in the kidneys, which boosts intestinal calcium absorption. Parathyroid hormone also lowers serum phosphate by decreasing its reabsorption in the kidneys, promoting its excretion. This coordinated control maintains calcium balance and phosphate homeostasis.

The thyroid gland contributes calcitonin, which can lower calcium by inhibiting bone resorption, but it plays a minor, non-primary role in adults. The adrenal glands and the pituitary gland do not regulate calcium and phosphate directly in the same central way as the parathyroid glands.