Calcium and vitamin D homeostasis in patients with heavy proteinuria

In the present study calcium and vitamin D metabolism was studied in fifty patients with daily urinary protein excretion exceeding 3 g/24 hours. A positive correlation was seen between serum albumin and ionized calcium concentration (r = 0.51, p < 0.001). All patients with normal renal function had their intact PTH within normal range, though seven of these 16 had serum ionized calcium below the lower margin of the reference range. The mean serum albumin was significantly lower in the seven patients with low serum ionized calcium when compared with those with serum ionized calcium above the lower normal margin (19 g/l vs. 30 g/l, respectively, p < 0.001), but there was no difference in plasma intact PTH (3.7 +/- 1.3 pmol/l vs. 3.1 +/- 0.7 pmol/l, p = 0.21, ns.). Serum 25OHD3 correlated negatively with the degree of proteinuria (r = -0.50, p < 0.001) and positively with serum albumin (r = 0.66, p < 0.001). Serum 1,25(OH)2D3 was related to serum 25OHD3 (r = 0.39, p < 0.01), but its association with serum phosphate, PTH, the degree of proteinuria and renal function did not reach statistical significance. In conclusion, a significant correlation between serum ionized calcium and albumin was observed, and in hypoalbuminemia hypocalcemia did not induce PTH response of expected magnitude. In patients with marked hypoalbuminemia a low measured ionized calcium does not have the same clinical impact as it would have in normoalbuminemia and it has to be interpreted cautiously. Secondly, abnormal regulation of 1,25(OH)2D3, and in particular its dependency on 25OHD3, was observed in patients with heavy proteinuria.