The skin contains 7-dehydrocholesterol. When it comes in
exposure to sunlight and ultraviolet rays 7-dehydrocholesterol is
converted into cholecalciferol.
Cholecalciferol is inactive.
Cholecalciferol is converted into vitamin D in two steps
In the liver cholecalciferol receives a hydroxyl group (= OH)
and is converted into 25OH Vitamin D calcidiol. Then in kidneys the
vitamin adds another hydroxyl group and becomes the active form
calcitriol. The transformation in kidney requires hormone
parathyroid hormone.