Which of the following best describes the typical lab findings in familial hypocalciuric hypercalcemia?

• A. Ca high, Phosphate low, PTH normal to slightly high
• B. Ca high, Phosphate high, PTH low
• C. Ca low, Phosphate low, PTH high
• D. Ca high, Phosphate normal, PTH suppressed

Answer: (A)

Familial hypocalciuric hypercalcemia comes from inactivating mutations in the calcium-sensing receptor (CaSR) on parathyroid and kidney cells, which shifts the calcium set-point upward. The body then behaves as if calcium is lower than it actually is, so PTH is not properly suppressed and the kidneys reabsorb more calcium. The net effect is mild to moderate hypercalcemia with PTH that is normal to mildly elevated, not suppressed. Phosphate tends to be low or inappropriately low-normal because PTH promotes phosphate excretion in the kidney. So the typical lab pattern is elevated calcium, relatively low phosphate, and PTH that is normal to mildly high. This contrasts with patterns seen in other disorders (for example, primary hyperparathyroidism often has higher PTH with low phosphate, not suppressed).