calcidiol
Calcidiol, also known as 25-hydroxyvitamin D [25(OH)D], is the principal circulating form of vitamin D in humans and the standard measure used to assess vitamin D status. It is produced in the liver, where cholecalciferol (vitamin D3) and ergocalciferol (vitamin D2) are hydroxylated by enzymes including 25-hydroxylase to form 25(OH)D. Calcidiol is transported in the blood largely bound to vitamin D–binding protein and has a relatively long circulating half-life of about two to three weeks. It reflects vitamin D obtained from sun exposure, diet, and supplements, and serves as a reservoir for the active hormone.
Calcidiol is not itself the biologically active vitamin D hormone. The active form is calcitriol (1,25-dihydroxyvitamin
In clinical practice, serum total 25(OH)D is the preferred marker of vitamin D status. Assays include immunoassays