The biologically active form of vitamin D is 1,25-dihydroxyvitamin D (1,25(OH)₂ D). Measurement of serum levels of 1,25(OH)₂ D should be considered upon suspicion of deficiency or excess of this form of the vitamin.

Reference ranges for 1,25(OH)2 D may be reported as either pg/mL or pmol/L. The molecular weight of 1,25(OH)2 D is approximately 416