Table 1.

Four prototypical physiological roles with response profile and biological targets for vitamin D and its metabolites for each.

Physiological Role	Response Profile	Target Tissues/Cells
defense against hypocalcemia	endocrine/paracrine endocrine/intracrine	bone kidney/gut
defense against hypercalcemia	paracrine/autocrine endocrine/intracrine	kidney bone
optimizing skeletal homeostasis	endocrine intracrine	bone/gut parathyroid
optimizing extraskeletal effects	intracrine paracrine	innate immune cells adaptive immune cells