Dynamics of PTH secretion in hemodialysis patients as determined by the intact and whole PTH assays

Background. Renal hyperparathyroidism is assessed by measurement of parathyroid hormone (PTH) levels. The intact PTH assay (I-PTH) not only reacts with 1-84 PTH but also with large, truncated fragments of non-1-84 PTH. Because the whole PTH assay (W-PTH) is specific for 1-84 PTH, non-1-84 PTH is determined by subtracting W-PTH from I-PTH values. These large circulating PTH fragments may exert a hypocalcemic effect by contributing to skeletal resistance to 1-84 PTH. Methods. The dynamic secretion of both 1-84 PTH and non-1-84 PTH was evaluated during the induction of hypo- and hypercalcemia in eight hemodialysis patients. Results. The basal ionized calcium concentration was 1.23 +/- 0.03 mmol/L at which time I-PTH, W-PTH, and non-1-84 PTH values were 276 +/- 78 pg/mL, 164 +/- 48 pg/mL, and 102 +/- 28 pg/mL, respectively. The induction of hypo- and hypercalcemic changes resulted in a sigmoidal response for all three PTH moieties, I-PTH, W-PTH, and non-1-84 PTH. During hypocalcemia, maximal values of W-PTH were greater than those of non-1-84 PTH. But during hypercalcemia, minimal values of W-PTH and non-1-84 PTH were similar. Neither the set points nor the basal/maximal ratios for W-PTH, I-PTH, and non-1-84 PTH were different. At the baseline ionized calcium concentration, the W-PTH (1-84 PTH)/non-1-84 PTH ratio was 1.53 +/- 0.15. Changes in ionized calcium resulted in a sigmoidal relationship with hypocalcemia, increasing this ratio to a maximum of 2.01 +/- 0.30 and hypercalcemia decreasing this ratio to a minimum of 1.18 +/- 0.15 (P < 0.01 vs baseline for both hypo- and hypercalcemia). Conclusion. Although acute changes in serum calcium produce similar secretory responses in 1-84 PTH and non-1-84 PTH, the secretory responses are not proportional for these PTH moieties. Changes in the serum calcium concentration modulate the ratio of 1-84 PTH/non-1-84 PTH in a sigmoidal pattern with hypocalcemia maximizing this ratio. Whether changes in the 1-84 PTH/non-1-84 PTH ratio specifically modulate the calcemic action and other biologic effects of 1-84 PTH remain to be determined.