ABSTRACT
The metabolic functions of vitamin D are accepted as that of a steroid hormone since vitamin D3 must be converted to the biologically active hydroxylated forms. 1α,25(OH)2D3 is transported to target organs where receptor binding occurs. The cellular-receptor of 1α,25(OH)2D3 is referred to as the vitamin D receptor (VDR). After activation, VDR alters transcription rates of target genes responsible for the biological responses of 1α,25(OH)2D3.4 The overall biological system is described as the vitamin D endocrine system.3 Biological responses include mobilization and accretion of calcium and phosphorous (bone), calcium and phosphorous absorption (intestine), and reabsorption of calcium and phosphorous (kidney). Other accepted roles for vitamin D related to the vitamin D endocrine system include osteoblast formation, fetus development, pancreatic function, neural function, immunity, and 1α,25(OH)2D3 mediated cellular growth and differentiation effects. The vitamin D endocrine system is described by Norman, 3 and the reader is encouraged to refer to this review.
