Figure 1.

The figure illustrates the structures of the three most familiar, clinically relevant vitamin D analogues, their primary in situ sites of generation, respective hydroxylating enzymes and reported concentrations in the bloodstream. While in humans, the circulatory levels of D₃ and 25(OH)D₃ are present in the nanomolar range, 1,25(OH)₂D is in the picomolar range: approximately a thousand-fold lesser concentration (numerical data from Hollis et al., added in the last two rows, [9]).