Which mineral abnormality is commonly elevated in CKD and drives secondary hyperparathyroidism?

• A. Calcium deficiency only
• B. Phosphate retention
• C. Sodium retention
• D. Potassium excretion

Answer: (B)

In CKD, the kidneys lose the ability to excrete phosphate effectively, so phosphate retention becomes elevated. This excess phosphate raises serum phosphate levels and lowers free calcium, because calcium binds with the phosphate. The drop in ionized calcium stimulates the parathyroid glands to secrete more parathyroid hormone, leading to secondary hyperparathyroidism. Additionally, reduced kidney function limits activation of vitamin D, which decreases intestinal calcium absorption and further drives PTH release. So the high phosphate level is the primary driver behind the development of secondary hyperparathyroidism in CKD. Potassium handling and sodium balance change with CKD but do not directly cause this parathyroid overactivity, and calcium can be affected as a consequence, not as the initial trigger.