PTH and Hypercalcemia: The minor contribution of 1,25(OH)D to hypercalcemia was also evident by an initial transient decline in serum calcium upon administration of phosphate, probably via inhibition of renal generation of 1,25(OH)D from 25OHD.(23, 24, 25) However, a consequent exaggerated rise in serum PTH with induction of a recurrence of hypercalcemia may confirm the major role of PTH in hypercalcemia.