Serum concentrations of vitamin D metabolites in different degrees of impaired renal function. Estimation of renal and extrarenal secretion rate of 24,25-dihydroxyvitamin D.

The serum levels of 25-hydroxycholecalciferol, 24,25-dihydroxycholecalciferol, and 1,25-dihydroxycholecalciferol were measured simultaneously in three groups of adults with different renal function: normal (n = 24), glomerular filtration rate (GFR) greater than 70 ml/min; moderately decreased (n = 15), GFR 5-35 ml/min, and severely decreased (n = 27), GFR less than 5 ml/min. 25-hydroxycholecalciferol was normal in both patient groups with decreased renal function. 1,25-dihydroxycholecalciferol was halved in the group with moderately decreased renal function (p less than 0.01) and severely decreased (almost undetectable) in the group with severely decreased renal function (p less than 0.01). In contrast, 24,25-dihydroxycholecalciferol was normal in the group with moderately decreased renal function and only halved (p less than 0.05) in the group with severely decreased renal function. These findings suggest extrarenal C-24 hydroxylase activity which may be stimulated by decreasing renal function, thus decreasing renal C-24 hydroxylase activity.