Primary hyperparathyroidism is characterized by which of the following laboratory findings?

• A. Decreased serum phosphate
• B. Decreased serum magnesium
• C. Increased hematocrit
• D. Increased bone density

Answer: (A)

Excess PTH in primary hyperparathyroidism raises calcium levels while driving phosphate out of the body. PTH acts on the proximal tubule to reduce phosphate reabsorption, causing renal phosphate wasting and a drop in serum phosphate. At the same time, bone resorption and increased calcium reabsorption raise serum calcium. So a low serum phosphate is a classic and defining lab finding.

Other options don’t fit the typical pattern: magnesium isn’t a defining change in this condition, hematocrit isn’t directly affected by PTH, and bone density is usually decreased due to bone loss from PTH-driven resorption, not increased.