The biologically active form of vitamin
D is 1,25 dihydroxyvitamin D, but the best indicator of
vitamin D status in individuals free of kidney disease is 25-
hydroxyvitamin D, the substrate for the renal and nonrenal
production of calcitriol, with a longer biological half-life
and a higher concentration than 1,25 dihydroxyvitamin D,
reflecting the total endogenous and exogenous production of
vitamin D