In addition, perturbations of the plasma ionized calcium concentration in vivo by intravenous infusions of calcium salts to induce hypercalcemia or Ca2+ chelators such as citrate or EGTA to induce hypocalcemia provoke rapid negative and positive changes in the serum PTH concentration respectively (Fox and Heath, 1981; Conlin et al., 1989; Schwarz et al., 1992). Here, PTH is linked to Hypocalcemia.