Action of vitamin D metabolites on PTH secretion in man

We have examined the effects of metabolites of vitamin D [25OHD3, 1,25(OH)2D3, 24,25(OH)2D3, and 25,26(OH)2D3] on serum calcium and iPTH in human deficient-D osteomalacia. The four metabolites decreased iPTH, but only for 1,25(OH)2D3 was a significant correlation between increase of serum calcium and decrease of iPTH observed. The 24,25(OH)2D3 and 25,26(OH)2D3 decreased iPTH despite a decrease of serum calcium at the beginning of treatment. The 25OHD decreased iPTH before increased serum calcium. These results could be interpreted as a direct effect of metabolites of vitamin D on PTH secretion. However, the conversion of other metabolites and the calcium concentration in parathyroid cells must be determined before this hypothesis can be accepted.