Which lab pattern best describes primary hyperparathyroidism?

Which lab pattern best describes primary hyperparathyroidism?

• A. Ca high; PTH high; Phosphate low; Vitamin D normal
• B. Ca high; PTH low; Phosphate high; Vitamin D high
• C. Ca low; PTH high; Phosphate normal; Vitamin D low
• D. Ca high; PTH normal; Phosphate high; Vitamin D high

Answer: (A)

High calcium with an inappropriately elevated PTH is the hallmark of primary hyperparathyroidism. PTH raises serum calcium by stimulating bone resorption and increasing renal calcium reabsorption, while it lowers serum phosphate by promoting phosphate excretion in the kidneys. The combination of high calcium and high PTH naturally comes with low phosphate because PTH-driven phosphaturia reduces phosphate levels. Active vitamin D can be increased by PTH, which boosts calcium absorption from the gut, but the circulating vitamin D status is often normal when measured, so “Vitamin D normal” fits with the typical lab pattern in primary hyperparathyroidism. This set of labs distinguishes it from other causes where PTH would be low (eg, PTH-independent hypercalcemia) or where phosphate or vitamin D patterns would contradict the PTH-driven process.

High calcium with an inappropriately elevated PTH is the hallmark of primary hyperparathyroidism. PTH raises serum calcium by stimulating bone resorption and increasing renal calcium reabsorption, while it lowers serum phosphate by promoting phosphate excretion in the kidneys. The combination of high calcium and high PTH naturally comes with low phosphate because PTH-driven phosphaturia reduces phosphate levels. Active vitamin D can be increased by PTH, which boosts calcium absorption from the gut, but the circulating vitamin D status is often normal when measured, so “Vitamin D normal” fits with the typical lab pattern in primary hyperparathyroidism. This set of labs distinguishes it from other causes where PTH would be low (eg, PTH-independent hypercalcemia) or where phosphate or vitamin D patterns would contradict the PTH-driven process.