CYP24A1 and hyperphosphatemia: Whilst decreases in serum calcium and/or phosphate levels stimulate this hydroxylation, hypercalcemia, hyperphosphatemia, and elevated 1,25(OH)2D concentrations decrease it, and stimulate the conversion of 25OHD and 1,25(OH)2D via cytochrome P450 24A1 (encoded by CYP24A1) to biologically less effective 24,25(OH)2D and 1,24,25(OH2)D in the renal tubules.