Effects of 24,25-dihydroxy-vitamin D3 on its plasma level in man

Small doses of 24,25-dihydroxy-vitamin D3 (24,25(OH)2D3; 2 micrograms daily) given by mouth increased plasma levels of 24,25(OH)2D3 several-fold in normal subjects and in patients with metabolic bone disease. When oral treatment was stopped, or following the intravenous administration of 24,25(OH)2D3, plasma disappearance of 24,25(OH)2D3 was slow (t1/2 approx. 16 days). This suggests that the endogenous production rate of 24,25(OH)2D3 is similar to that of 1,25(OH)2D3 (less than 2 micrograms daily) but that its metabolic clearance is much slower. The slow metabolic clearance of 24,25(OH)2D3 may explain why plasma levels of this metabolite are about 100 times higher than those of 1,25(OH)2D3.