CYP24A1 and Hypercalcemia: Subsequently, the low levels of serum phosphate and FGF23 induce increases in CYP27B1 expression and 1a-hydroxylase activity; while inhibiting CYP24A1 expression and 24-hydroxylase activity, which have the combined effect of increasing 1,25-(OH)2D3 and hypercalcaemia [8].