PTH and Hypercalcemia: In this regard, Hollemberg et al. [14] and Perez et al. [13] proposed the following theoretical explanations to the mechanism of inappropriately low PTH level in the PHPT cases observed: a circulating inhibitor of PTH; the pulsatile secretion of the hormone; an abnormal PTH molecule with increased biologic activity; increased peripheral tissue sensitivity to normal PTH; or the presence of another mediator of hypercalcemia (eg, PTHrP).