PTH and Hypocalcemia: In the face of hypocalcemia, calcium sensor-receptors (CaSR) in the parathyroid cells induce PTH synthesis, which stimulates the CYP27B1 gene encoding 1 alpha-hydroxylase and synthesizes 1,25(OH)2D. The latter provides negative feedback, inhibiting the CYP27B1 and decreasing PTH levels, directly suppressing its gene transcription from parathyroids and indirectly suppressing it by increasing calcemia (8,18).