A 72-year-old female with recurrent nephrolithiasis has hypercalcemia and decreased phosphate. What is the most likely diagnosis?

• A. Pheochromocytoma
• B. Adrenal Insufficiency
• C. Hyperparathyroidism
• D. Vitamin D Deficiency

Answer: (C)

Hypercalcemia with decreased phosphate points to excess parathyroid hormone activity, as seen in primary hyperparathyroidism. PTH raises serum calcium by increasing bone resorption and renal calcium reabsorption, while promoting phosphate excretion in the kidney, leading to low serum phosphate. The elevated calcium also predisposes to nephrolithiasis, matching this patient’s recurrent stones. PTH also drives production of active vitamin D, boosting intestinal calcium absorption and further elevating calcium levels. The other conditions don’t fit this pattern: pheochromocytoma doesn’t cause a hypercalcemia–hypophosphatemia profile, adrenal insufficiency has different electrolyte disturbances, and vitamin D deficiency typically causes hypocalcemia rather than hypercalcemia.