What mechanism drives secondary hyperparathyroidism in CKD?

• A. Significant parathyroid hyperplasia; prolonged hypocalcemia may make PTH autonomous
• B. Decreased parathyroid mass
• C. Increased calcitonin production
• D. Elevated vitamin D synthesis

Answer: (A)

Chronic kidney disease disrupts mineral balance in a way that keeps the parathyroid glands under continuous stimulation. The kidney’s reduced ability to excrete phosphate leads to hyperphosphatemia, which binds calcium and lowers free calcium levels. At the same time, damaged kidneys have less 1α-hydroxylase activity, so less active vitamin D is produced, reducing intestinal calcium absorption. The combination of low calcium and high phosphate drives persistent PTH secretion, and the glands respond by increasing in number and size (hyperplasia). Over time, this hyperplasia can make PTH secretion less dependent on circulating calcium, approaching autonomous activity. Thus, significant parathyroid hyperplasia driven by chronic hypocalcemia and hyperphosphatemia is the main mechanism of secondary hyperparathyroidism in CKD. The other options don’t fit because CKD typically does not cause decreased parathyroid mass, increased calcitonin production, or elevated vitamin D synthesis; in CKD, calcitriol production is reduced, not increased.