DIRECT INVIVO COMPARISON OF CALCIUM-REGULATED PARATHYROID-HORMONE SECRETION IN NORMAL VOLUNTEERS AND PATIENTS WITH SECONDARY HYPERPARATHYROIDISM

The regulation of PTH secretion by calcium is altered in patients with primary hyperparathyroidism. A similar disturbance may occur in secon dary hyperparathyroidism, but direct in vivo comparisons of PTH secret ion in normal subjects and those with secondary hyperparathyroidism ha ve not been made. Thus, 13 patients with end-stage renal failure and s econdary hyperparathyroidism and 20 healthy volunteers underwent dynam ic tests of PTH secretion. Changes in ionized calcium were induced by 2-h iv infusions of calcium gluconate or sodium citrate on consecutive days, and the sigmoidal relationship between serum ionized calcium an d PTH levels was examined. During sodium citrate infusions, serum ioni zed calcium levels decreased by 0.21 +/- 0.04 and 0.20 +/- 0.05 mmol/L , respectively (mean +/- SD), in normal volunteers and dialyzed patien ts (P = NS). Serum PTH levels rose from 27 +/- 7 to 107 +/- 33 pg/mL i n controls and from 480 +/- 238 to 859 +/- 412 pg/mL in dialyzed subje cts; thus, maximum PTH levels were 396% of preinfusion values in norma l subjects, but only 79% greater than baseline values in dialyzed pati ents (P < 0.001). During the first 30 min of calcium infusions, the in crease in serum ionized calcium did not differ between groups, but PTH levels fell more rapidly in normal volunteers; values were 24% of pre infusion levels in controls, but only 56% of the baseline in dialyzed patients (P < 0.01) after 30 min. Minimum PTH levels were attained aft er 50 min of calcium infusion in normal volunteers and after 70 min in dialyzed patients. The derived values for set-point were 1.21 +/- 0.0 4 and 1.24 +/- 0.06 mmol/L, respectively, in control and dialyzed subj ects (P = NS). These results do not support the contention that the se t-point for calcium-regulated PTH secretion is greater than normal in patients with secondary hyperparathyroidism due to end-stage renal dis ease.