The vitamin D system: a view from basic science to the clinic.

Vitamin D produced in the skin and absorbed in the small intestine must be modified metabolically before it can function. It is ultimately converted to a hormone in the kidney that stimulates intestine, bone and kidney to mobilize calcium and phosphorus. This results in normal bone development and normal neuromuscular function. The vitamin D hormone appears to act by a nuclear mechanism to facilitate a target organ response. Finally the vitamin D hormone is produced in response to the need for calcium and phosphorus. The calcium need is interpreted by the parathyroid gland that in turn secretes parathyroid hormone. The parathyroid hormone stimulates production of the vitamin D hormone. This constitutes the vitamin D endocrine system that plays an important role not only in calcium homeostasis but in phosphate homeostasis and in calcium economy of the body. A number of disease states including hypoparathyroidism, pseudohypoparathyroidism, renal osteodystrophy, certain types of vitamin D-resistant rickets and osteoporosis can in part be related to disturbance in the vitamin D endocrine system. Thus measurement of the vitamin D hormone and its precursor will be of great value in diagnosis of metabolic bone disease and most importantly, the availability of new vitamin D compounds will play an important role in the treatment of these bone diseases.