PTH inhibition rate is useful in the detection of early-stage primary hyperparathyroidism.

Objective This study was to establish biochemical thresholds for the intravenous calcium suppression test in the early diagnosis of primary hyperparathyroidism (PHPT). Design and methods One hundred and thirty-three patients were divided into three groups: Group 1: surgically proven hypercalcemic PHPT, Group 2 surgically proven mild PHPT, and Group 3: normocalcemia with elevated serum PTH levels. Intravenous calcium suppression tests were performed in Groups 2 and 3 as well as in 20 controls with normal serum calcium and PTH concentrations. Results The serum PTH inhibition rate (PTH-IR) was less pronounced in Group 2 compared with Group 3 (P < 0.001) and the controls (P < 0.001). Receiver operating characteristic curve analysis suggests that a serum calcium level higher than 2.43 mmol/L and a PTH-IR less than 73% may differentiate Group 2 from normal controls. Conclusion It is quite useful to combine serum calcium levels with the PTH-IR to identify patients at early stage of PHPT, even in the presence of vitamin D deficiency.