PTH and primary hyperparathyroidism: A previous study found 30 cases (0.6%) with normal serum calcium and inappropriately high serum PTH (normocalcemic primary hyperparathyroidism) in 5202 subjects, using the diagnostic criteria including serum calcium 2.50-2.60 mmol/l with PTH ≥ 35 ng/L and calcium < 2.50 mmol/l with PTH > 55 ng/L [58].