Notes:
To get everyone on the same page, I have made a rather simplified summary of vitamin D metabolism.
Essentially, most animals obtain vitamin D by making it in their skin or by eating it.
In the skin, 7-dehydrocholesterol is transformed by exposure to ultraviolet light into pre-vitamin D. This is then absorbed into circulation (the blood). The other path is via consumption of Vitamin D3 (sources include fish liver and egg yolks) or Vitamin D2 (plant sources). The ingested vitamin D (D2 or D3) is absorbed from the small intestine into the bloodstream.
From the blood, the vitamins (made in the skin or from the diet) travel to the liver where they are converted by a hydroxylase enxzyme to 25-hydroxyvitamin D (25-OH-D or calcidiol). 25-OH-D is the main circulating form of the vitamin and can be measured as an indicator of overall vitamin D status. A subsequent hydroxylation reaction can occur in the kidney to produce 1,25-dihydroxyvitamin D (1,25-OH-D or calcitriol). The resulting metabolite, 1,25-OH-D, is many times more potent than 25-OH-D which may explain why this reaction is tightly regulated whereas that in the liver is not.