Pathways of vitamin D synthesis

Metabolic activation of vitamin D to calcitriol. The conversion of calcidiol to calcitriol in the kidney is tightly regulated by numerous factors, including PTH, FGF23, and serum phosphorus concentrations.

The primary action of calcitriol is to promote intestinal calcium and phosphorus absorption; it also has minor, direct effects on bone resorption and kidney calcium retention. The net effect on serum and urinary calcium and phosphorus concentrations depends on numerous other factors and feedback loops.