What metabolic complication is commonly seen in chronic kidney disease due to phosphate retention and decreased calcium?

• A. Hyperparathyroidism without renal osteodystrophy
• B. Vitamin D toxicity
• C. Secondary hyperparathyroidism leading to renal osteodystrophy
• D. Renal osteitis fibrosa without PTH changes

Answer: (C)

When kidney function declines, phosphate isn’t excreted effectively and tends to rise in the blood. High phosphate plus lower levels of active vitamin D reduce calcium absorption from the gut, so serum calcium falls. The parathyroid glands respond to this hypocalcemia (and hyperphosphatemia) by increasing parathyroid hormone secretion—secondary hyperparathyroidism. The elevated PTH drives bone turnover in an attempt to release calcium, which over time leads to renal osteodystrophy, the bone disease commonly seen with CKD. Vitamin D toxicity isn’t the issue here because the problem is insufficient activation of vitamin D, not excess. Primary hyperparathyroidism is a different disorder where the parathyroid glands themselves are the primary source of excess PTH. Renal osteitis fibrosa describes the bone changes from secondary hyperparathyroidism and involves high PTH levels.