Current clinical applications of vitamin D metabolite research.

Vitamin D2 or D3 are prohormones that are converted in the liver and kidney to three metabolites, all having biological activity. the most prevalent circulating metabolite is 25(OH)D, and when its level falls, osteomalacia and rickets result. The second most prevalent metabolite is 24,25(OH)2D which may be a mineralizing hormone and which may impair PTH secretion, although wide controversy prevails over its function. Calcitriol circulates at levels a thousand-fold less than 25(OH)D and is the most active metabolite in terms of calcium absorption from the gut and mobilization of calcium and phosphate from bone. Clinical defects in this system result from abnormalities of vitamin D metabolism or from end-organ resistance to these hormones. Many disorders of demineralization can now be understood more clearly, and treated with a greater understanding of the underlying pathophysiology. Nonetheless, many new questions about the role of various vitamin D metabolites have now arisen.