<prompt> The blood levels of 1,25 dihydroxycholecalciferol is positively regulated by:-PTHCalciumMagnesium25 OH cholecalciferol  <response>: PTH - PTH is necessary for conversion of 25-hydroxy chole calciferol to 1,25 - dihydroxy cholecalciferol: Calcium: Plasma concentration of 1,25 OH2 D3 is inversely affected by Ca2 concentration in plasma. The rate of PTH secretion is greatly suppressed when plasma Ca2 concentration rise above 9-10 mg100gm If Ca2 concentration below this level - es conversion of 25 hydroxycholecalciferol to 1,25 dihydroxycholecalciferol in the kidney At higher calcium concentration when PTH is suppressed the 25 Hydroxycholecalciferol is conveed to a different component - 24, 25- Dihydroxy cholecalciferol that has almost no vitamin D defect 