The fasting phosphate concentration is primarily regulated by which endocrine gland?

• A. Pancreas
• B. Skeleton
• C. Parathyroid glands
• D. Small intestine

Answer: (C)

The main regulator of fasting phosphate concentration is the parathyroid glands. When phosphate levels rise or calcium is low, the parathyroid glands secrete parathyroid hormone (PTH). PTH acts on the kidneys to reduce phosphate reabsorption in the proximal tubule, leading to increased renal phosphate excretion and a drop in serum phosphate. It also stimulates the activation of vitamin D, which increases intestinal phosphate absorption, but the dominant effect that sets fasting phosphate levels is the renal excretion controlled by PTH. The skeleton serves as a large phosphate reservoir and responds to PTH, but it doesn’t govern phosphate levels as an endocrine regulator. The pancreas and small intestine are not the primary regulators of fasting phosphate balance.