The regulation of PTH secretion by calcium is altered in patients with PHPT [10,15,16]. It was demonstrated that the reduction in PTH levels after either the oral or intravenous load of calcium were significantly less in patients with PHPT [12], [17–19]. Similar results were also observed in the current study: PTH-IR was markedly diminished in Group 2 compared with the control group (41 vs. 80%).All these findings clearly confirmed that in PHPT, the PTH-calcium curve is shifted to the right and the set-point for calcium is increased.A calcium sensing error in abnormal parathyroid cells and decreased calcium sensing receptor (CaSR) or G protein (Gq) expression may be responsible for such changes [20,21].