BOC Clinical Chemistry Practice Test

High calcium with high parathyroid hormone and low phosphate points to overactivity of the parathyroid glands. Parathyroid hormone raises serum calcium by stimulating bone resorption, increasing calcium reabsorption in the kidneys, and boosting production of active vitamin D to enhance intestinal calcium absorption. It also decreases renal phosphate reabsorption, lowering serum phosphate. This combination—elevated calcium, reduced phosphate, and elevated PTH—is classic for primary hyperparathyroidism, usually due to a parathyroid adenoma.

Hypoparathyroidism would show low PTH with low calcium and high phosphate. Nephrosis and steatorrhea don’t produce this pattern; they don’t cause a sustained rise in calcium with high PTH.