In CKD, phosphate tends to accumulate due to reduced renal excretion. How does elevated phosphate affect calcium levels?

• A. Decreased renal excretion leading to hyperphosphatemia, which binds calcium and stimulates PTH.
• B. Increased intestinal absorption of phosphate causing hypercalcemia.
• C. Increased phosphate production by bone causing hypercalcemia.
• D. Reduced phosphate intake leading to hypophosphatemia with high PTH.

Answer: (A)

In CKD, phosphate builds up because the kidneys can’t excrete it effectively. That excess phosphate binds calcium in the blood, forming calcium-phosphate complexes and reducing the amount of free (ionized) calcium. The drop in serum calcium then triggers the parathyroid glands to release more parathyroid hormone, leading to secondary hyperparathyroidism. So, elevated phosphate lowers circulating calcium and stimulates PTH release as the body tries to compensate, even though kidney dysfunction limits the ability to correct calcium levels.