Basic and Clinical Concepts Related to Vitamin D Metabolism and Action

(First of Two Parts) A NOVEL sterol hormone that is undoubtedly a major physiologic regulator of mineral metabolism in man, 1α,25-dihydroxyvitamin D (1,25(OH)₂D), is produced from vitamin D in response to situations of calcium and phosphorus need, with the final enzymatic reaction occurring in the kidney. A complex and elegant biochemical mechanism is thought to function at the pivotal kidney site to modulate the biosynthesis of this most active form of vitamin D. In concert with other calcitropic principles, such as parathyroid hormone (PTH) and calcitonin, the l,25-(OH)₂D hormone mediates calcium and phosphorus metabolism at target.