[Does 24,25 dihydroxycholecalciferol have a physiological and pathophysiological role?]

The authors review recent experimental and human data concerning the potential physiological and pathophysiological role of 24,25 (OH)2D3, the dihydroxylated metabolite of vitamin D which is synthetisized with preference over 1,25 (OH)2D3 in organisms that have been replenished with vitamin D. For the major known effects of vitamin D such as stimulation of intestinal absorption and bone resorption of calcium and phosphorus, 24,25 (OH)2D3 is less effective than the 1,25 (OH)2D3 metabolite and consequently of lesser physiological importance. Some recent in vitro experiments have shown, however, that 24,25 (OH)2D3 intervenes in the stimulation of proteoglycan synthesis, inhibition of PTH, vitamin A and heparin induced resorption, whereas 1,25 (OH)2D3 does not. Although there is controversy as to its direct inhibitory effect on secretion of PTH, it seems to act with 1,25 (OH)2D3 to prevent hyperplasia of parathyroids in vitamin D deficient chicken. From a pathophysiological point, the presence of 24,25 (OH)2D3 seems vital to allow normal bone formation and mineralisation and possibly to counteract excessive bone resorption.