Relative Value of 25(OH)D and 1,25(OH)2D Measurements

VITAMIN D STATUS and metabolism is characterized by many circulating metabolites. Serum 25-hydroxyvitamin D [25(OH)D], 1,25-dihydroxyvitamin D [1,25(OH)2D], and 24,25-dihydroxyvitamin D [24,25(OH)2D] have been assessed in many epidemiological studies. The first two metabolites are of major interest for the assessment of vitamin D status and evaluation of vitamin D metabolism. The major circulating metabolite is 25(OH)D, whereas the active metabolite is 1,25(OH)2D, which is responsible for most actions of vitamin D. The production of 25(OH)D may be limited in severe liver disease. The hydroxylation to 1,25(OH)2D is more often restricted by kidney disease or decrease of renal function, as occurs with aging. The production of 24,25(OH)2D is induced by 1,25(OH)2D and is part of the catabolic pathway. The characteristics of both metabolites are presented in Table 1. The concentration of 25(OH)D is almost 1000-fold of that of 1,25(OH)2D, and the half-life of 25(OH)D is much longer, implying that its concentration is more stable. Serum 25(OH)D is decreased by thyroid hormone, anticonvulsants, cholestyramine, and orlistat, but otherwise stable. The concentration of 1,25(OH)2D is directly influenced by serum calcium and phosphate, calcium intake, and immobility. Serum 1,25(OH)2D is increased by PTH and PTH-related peptide, prolactin, estradiol, testosterone, prostaglandins, and bisphosphonates and decreased by corticosteroids, phosphatonin, ketoconazole, heparin, and thiazides. The stimulation by other hormones and negative feedback loops classify it as a hormone.