In chronic kidney disease, how are serum calcium, phosphate, and PTH typically affected?

• A. Calcium low; Phosphate high; PTH high
• B. Calcium high; Phosphate low; PTH low
• C. Calcium normal; Phosphate normal; PTH high
• D. Calcium high; Phosphate high; PTH high

Answer: (A)

In chronic kidney disease, the kidneys’ ability to regulate calcium and phosphate is disrupted. They cannot excrete phosphate effectively, so serum phosphate rises. Higher phosphate levels bind calcium and reduce the amount of free ionized calcium. At the same time, damaged kidneys produce less active vitamin D, which lowers intestinal absorption of calcium. The resulting drop in serum calcium stimulates the parathyroid glands to release more PTH, leading to secondary hyperparathyroidism. The net pattern you typically see is low calcium, high phosphate, and high PTH. This PTH response attempts to restore calcium levels by mobilizing calcium from bone, but it cannot fully correct the imbalance because the kidney’s phosphate handling and vitamin D activation remain impaired. While calcium may be normal early on in some patients, the classic CKD picture is low calcium, high phosphate, and high PTH.