Physiology of 24,25-dihydroxyvitamin D₃ in normal human subjects

The authors determined the metabolic clearance and production rates of 24,25-dihydroxyvitamin D₃ in four normal healthy adults. They also examined the excretion of radioactivity in stool, urine, and bile after the intravenous administration of 24,25-[³H]dihydroxyvitamin D₃ to human subjects. 24,25-Dihydroxyvitamin D₃ is rapidly cleared from the plasma with a half-life of ~ 390 ± 25 min (mean ± SE). The metabolic clearance rate of 24,25-dihydroxyvitamin D₃ was 9.2 ± 1.5 liters/day with a production rate of 26.4 ± 7.2 μg/day (mean ± SE). Within 1 day 13.0 ± 4.2% (mean ± SE) of the administered dose had appeared in the stool: by day 7, 48.8 ± 2.7% of the dose had appeared in the feces. Within 24 hr, 6.4 ± 0.8% of the administered dose appeared in the urine; 7.4 ± 1.8% of the dose had appeared in the urine within 2 days. The biliary excretion of 24,25-dihydroxyvitamin D₃ was studied in two subjects. By 8 h, 15.3 ± 1.3% of the administered dose had appeared in the bile. The metabolites present in bile, feces, and urine were much more polar than 24,25-dihydroxyvitamin D₃. These results demonstrate that 24,25-dihydroxyvitamin D₃ is rapidly cleared from plasma and is excreted in the feces (probably via the bile) and urine of normal human subjects.