What is the typical laboratory pattern in primary hyperparathyroidism?

• A. Hypercalcemia with suppressed PTH and hypocalciuria.
• B. Hypocalcemia with low PTH.
• C. Hypercalcemia with inappropriately normal or elevated PTH, often with hypercalciuria.
• D. Hypercalcemia with low phosphate.

Answer: (C)

In primary hyperparathyroidism, the parathyroid glands overproduce PTH despite elevated calcium. Normally, high calcium should suppress PTH, but in this condition the response is inappropriately normal or elevated. That persistent PTH elevation drives more calcium into the blood, so you see hypercalcemia. PTH also causes phosphate wasting in the kidney, so phosphate tends to be low, and it increases calcium excretion in the urine, leading to hypercalciuria. This combination—high calcium with inappropriately normal or elevated PTH and often hypercalciuria—best fits the typical lab pattern for primary hyperparathyroidism. It helps distinguish PHPT from other causes of hypercalcemia (where PTH would be suppressed) and from conditions like familial hypocalciuric hypercalcemia, where urinary calcium is low despite high calcium.