New developments in the vitamin D endocrine system.

Vitamin D must be altered first by 25-hydroxylation in the liver and subsequently by 1-hydroxylation in the kidney in order to stimulate intestinal calcium transport, bone calcium mobilization, intestinal phosphate transport, and renal tubular reabsorption of calcium. Other pathways of vitamin D metabolism are described, and evidence is presented which indicates that only 25- and 1-hydroxylation pathways are activation mechanisms. The 1 alpha-hydroxylation step is regulated by the need for calcium and the need for phosphorus. The vitamin D hormone, 1,25-dihydroxyvitamin D3, appears to act in target tissues by virtue of interacting with a receptor appearing in the nucleus and eliciting transcription of specific genetic information that codes for calcium and phosphorus transport proteins. Diseases of calcium and phosphorus metabolism in which there is a disturbance of the vitamin D endocrine system and the use of the vitamin D metabolites are discussed.