Which mineral abnormalities drive secondary hyperparathyroidism in chronic kidney disease?

• A. Hyperphosphatemia and hypocalcemia (with reduced 1,25-dihydroxyvitamin D)
• B. Hypercalcemia and hyperphosphatemia
• C. Hypophosphatemia and hypercalcemia
• D. Normal phosphate and calcium with high vitamin D

Answer: (A)

In chronic kidney disease the kidneys lose the ability to excrete phosphate and to activate vitamin D. This leads to high phosphate levels (hyperphosphatemia) and low levels of active vitamin D (1,25-dihydroxyvitamin D). The excess phosphate binds calcium, reducing the amount of free calcium in the blood and producing hypocalcemia. At the same time, reduced kidney function means less calcitriol, so intestinal calcium absorption is decreased, which compounds the fall in calcium. This combination—hyperphosphatemia, hypocalcemia, and low active vitamin D—stimulates the parathyroid glands to release more parathyroid hormone, producing secondary hyperparathyroidism. Hypercalcemia would counteract this by suppressing PTH, and other patterns like hypophosphatemia or normal phosphate with high vitamin D do not drive the same PTH response in CKD.