A 60-year-old woman with elevated ionized calcium, decreased serum inorganic phosphate, and elevated urine calcium and phosphate is most consistent with which diagnosis?

• A. Primary hyperparathyroidism
• B. Vitamin D deficiency
• C. Hypoparathyroidism
• D. Paget disease

Answer: (A)

The key idea is how parathyroid hormone (PTH) regulates calcium and phosphate in the body. PTH raises serum calcium by stimulating bone resorption and increasing calcium reabsorption in the kidneys, while it lowers serum phosphate by promoting phosphate excretion in the proximal tubule. The kidneys respond by dumping more calcium and more phosphate into the urine, so you see both elevated urinary calcium and urinary phosphate. When you put that together with a high ionized calcium and a low serum inorganic phosphate, it points to excessive PTH driving these changes—classic for primary hyperparathyroidism.

Vitamin D deficiency would typically lower calcium (and often phosphate) and provoke only a secondary rise in PTH, not the hypercalcemia seen here. Hypoparathyroidism would cause low calcium and high phosphate. Paget disease mainly alters bone turnover and usually does not produce this pattern of calcium and phosphate abnormalities.