[3][4] This enzymatic 25(OH)D-1α-hydroxylase reaction is performed exclusively by CYP27B1, which is highly expressed in the kidneys where it is principally regulated by parathyroid hormone, but also by FGF23 and calcitriol itself.

[3] This enzymatic reaction is performed by CYP24A1 which is expressed in many vitamin D target tissues including kidney, and is induced by calcitriol.

[4][15] Patients with osteoporosis, chronic kidney disease, malabsorption, obesity, and some other infections may be at greater risk for being vitamin D-deficient and so are more likely to have this test.

[15] Physicians may advise low risk patients to take over-the-counter vitamin D supplements in place of having screening.

[11] The normal range varies widely depending on several factors, including age and geographic location.

A broad reference range of 20 to 150 nmol/L (8-60 ng/mL) has also been suggested,[16] while other studies have defined levels below 80 nmol/L (32 ng/mL) as indicative of vitamin D deficiency.

[6] A review of the results of nine randomized control trials which compared oral doses of both, found that calcifediol was 3.2-fold more potent than cholecalciferol.

[20] In 2016, the FDA approved a formulation of calcifediol (Rayaldee) 60 microgram daily as a prescription medication to treat secondary hyperparathyroidism in patients with chronic kidney disease.

[22] Studies are ongoing comparing the effects of calcifediol with other forms of vitamin D including cholecalciferol in prevention and treatment of osteoporosis.

There has been some success in using dietary calcifediol supplementation in salmonids to improve growth and food conversion ratio.