CYP24A1 and vitamin D deficiency: Low or undetectable concentrations of 24,25(OH)2D3 might indicate CYP24A1 deficiency or functional vitamin D deficiency where all 25(OH)D is needed to maintain an adequate cellular supply with active 1,25(OH)2D. Another limitation of previous studies is the measurement of 25(OH)D with different immunoassays, which have shown variable accuracy in comparison to validated LC-MS/MS reference methods [24, 25].