In CKD, phosphate retention in CKD leads to decreased calcium, stimulating parathyroid hormone release causing bone disease. This statement is:

• A. Calcium remains constant; no effect on PTH
• B. Phosphate retention increases calcium levels
• C. Phosphate retention in CKD leads to decreased calcium, stimulating parathyroid hormone release causing bone disease
• D. PTH decreases with phosphate retention

Answer: (C)

In CKD, the kidneys lose the ability to excrete phosphate, so phosphate builds up in the blood. The extra phosphate binds calcium and lowers the level of free (ionized) calcium. At the same time, reduced kidney function means less activation of vitamin D, which decreases intestinal calcium absorption. The resulting hypocalcemia triggers the parathyroid glands to release more PTH, leading to secondary hyperparathyroidism. Chronic elevated PTH drives bone turnover and bone disease, often termed renal osteodystrophy. That cascade—phosphate retention lowering calcium and provoking PTH release, with subsequent bone changes—is exactly what this statement describes.

Calcium does not stay constant in this scenario, and phosphate retention does not raise calcium. PTH does not decrease with phosphate retention; it increases in response to hypocalcemia.