[Regulation of vitamin D biosynthesis by calcium]

Vitamin D is a secosteroid that is made in the skin by the action of sunlight. Vitamin D is biologically insert and must undergo two successive hydroxylations in the liver and kidney to become the biologically active vitamin D [1alpha,25 (OH)(2)D]. Recently it was appreciated that 1alpha,25 (OH)(2)D not only regulates calcium but also is capable of inhibiting the proliferation and inducing terminal differentiation of variety of cells. The renal production of 1alpha,25 (OH)(2)D is tightly regulated by serum calcium levels through the action of parathyroid hormone.