Improvement of secondary hyperparathyroidism and reduction of the set point of calcium after intravenous calcitriol.

In this study we evaluated the effect of intravenous calcitriol on parathyroid function and ionized calcium/PTH sigmoidal curve obtained during low and high calcium hemodialysis in 10 patients with osteitis fibrosa whose secondary hyperparathyroidism was refractory to conventional therapy. After four months of intravenous calcitriol, serum ionized calcium increased from 1.28 +/- 0.08 to 1.37 +/- 0.11 mmol/liter (P < 0.001), serum phosphate from 1.54 +/- 0.18 to 1.79 +/- 0.4 mmol/liter (P = NS), serum calcitriol from 16.7 +/- 9.9 to 34.3 +/- 6.4 pg/ml (P < 0.001), while alkaline phosphatase decreased from 366 +/- 340 to 226 +/- 180 IU/liter (P < 0.05), osteocalcin from 46.4 +/- 20 to 34.5 +/- 15.3 ng/ml (P < 0.05) and basal intact PTH from 1069 +/- 700 to 305 +/- 270 (P < 0.01). Basal PTH started to decrease after one month of treatment prior to the increase in the ionized calcium. Because of hypercalcemia, the dialysate calcium was decreased from 1.75 to 1.5 mmol/liter in three of five patients on hemodialysis and calcium-containing solutions were substituted by calcium-free replacement fluids in four of five patients on hemodiafiltration. Calcitriol dose at the first month of therapy was 5.6 +/- 0.8 micrograms/week, but successively it was decreased because of hypercalcemia to a final dose of 3.6 +/- 1.3 micrograms/week. After intravenous calcitriol the ionized calcium/PTH sigmoidal curve shifted to the left and downward. Maximally stimulated PTH and maximally inhibited PTH obtained during low and high calcium dialysis significantly decreased as did the ratio of basal PTH/PTHmax and the set point of calcium.(ABSTRACT TRUNCATED AT 250 WORDS)