Lack of relationship between parathyroid hormone and 1,25-dihydroxyvitamin D in chronic renal failure

In the present study, concentrations of parathyroid hormone (PTH), determined by an intact PTH assay and a midregion/C-terminal PTH assay, 1,25-dihydroxyvitamin D [1,25(OH)2D3], ionized calcium and phosphate were measured in 15 patients with a stable creatinine clearance (Ccr) of 21.2 +/- 14.4 ml/min (mean +/- SD; group 1) and in 10 patients with a Ccr regularly undergoing hemodialysis (group 2, Ccr not measured). In group 1, the mean concentration of 1,25(OH)2D3 was significantly increased compared with the level in group 2, whereas no differences were found concerning the concentrations of intact PTH, midregion/C-terminal PTH, ionized calcium and phosphate. In group 1, the PTH concentration correlated inversely with ionized calcium concentration and Ccr, which in turn, was directly correlated. The concentration of 1,25(OH)2D3 correlated inversely with phosphate concentration, but did not correlate with either PTH or ionized calcium concentrations. In group 2 no correlation was found between any of the biochemical variables. The data demonstrate that in patients with stable renal failure, the concentration of ionized calcium still regulates PTH secretion but other variables such as parathyroid cell mass and setpoint may interfere with the interrelation. The elevated concentration of phosphate in renal failure may override PTH as a regulator of the renal 1,25(OH)2D3 formation. The lack of correlation in the hemodialyzed patients may be attributed to extrarenal production of 1,25(OH)2D3, reduced binding of 1,25(OH)2D3 to parathyroid tissue or the major changes in calcium homeostasis caused by the hemodialysis.