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

ObjectiveThis study was to establish biochemical thresholds for the intravenous calcium suppression test in the early diagnosis of primary hyperparathyroidism (PHPT).Design and methodsOne 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.ResultsThe 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.ConclusionIt 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.