Activation of Vitamin D to Calcitriol

(Activation of Vitamin D:)Step Three

- CALCIDIOL circulates in the blood: as it passes through the blood vessels of the kidney, it is converted to CALCITRIOL by kidney enzymes (when another -OH group is added) - CALCITRIOL is the active form of Vitamin D3 - The presence of parathyroid hormone increases the rate of this final enzymatic step in the kidney. Thus, greater amounts of CALCITRIOL are formed when PTH is present.

(Activation of Vitamin D:)Step One

- UV light converts the precursor molecule in keratinocytes of the skin to Vitamin D3 (CHOLECALCIFEROL), which is released into the blood. - Vitamin D3 also is absorbed from the small intestine into the blood from the diet

(Activation of Vitamin D:)Step Two

- Vitamin D3 circulates throughout the blood. As is passes through the blood vessels of the liver, it is converted by liver enzymes to CALCIDIOL by the addition of a hydroxyl group (-OH). - Both steps 1 & 2 occur continuously with limited regulation