Which condition features the hallmark biochemical signature of secondary hyperparathyroidism in CKD?

• A. Low PTH with hypercalcemia
• B. Elevated PTH with hyperphosphatemia, decreased 1,25(OH)2D, and often low-normal calcium
• C. Low PTH with hypophosphatemia
• D. Elevated PTH with hypercalcemia

Answer: (B)

In CKD, the kidney’s ability to excrete phosphate and to activate vitamin D is impaired. This leads to high phosphate and low levels of active vitamin D (1,25-dihydroxyvitamin D). The elevated phosphate and reduced calcitriol decrease calcium absorption, so serum calcium tends to be low or low-normal. The fall in calcium and direct stimulus from phosphate retention drive the parathyroid glands to secrete more PTH, resulting in secondary hyperparathyroidism. The hallmark biochemical signature in this scenario is elevated PTH with hyperphosphatemia, decreased 1,25(OH)2D, and often low-normal calcium, which differentiates it from primary hyperparathyroidism (high calcium) and other calcium-phosphate disorders.