Vitamin D, either from food or dermal production, undergoes two hydroxylation; first metabolized to 25-hydroxy vitamin D [25(OH)D] in the liver by such enzymes as CYP2R1, then catalyzed by CYP27B1 to 1,25-dihydroxy vitamin D [1,25(OH)2D] in the kidney, which is the active form of vitamin D.(4) CYP27B1 activity is under the strict control; i.g. induction by parathyroid hormone (PTH) and suppression by hypercalcemia or increased serum level of 1,25(OH)2D (Fig. 1). Here, PTH is linked to Hypercalcemia.