Differences in the dynamics of parathyroid hormone secretion in hemodialysis patients with marked secondary...

Hemodialysis patients with predialysis intact parathyroid hormone (PTH) levels of more than 500 pg/mL are generally considered to have marked secondary hyperparathyroidism. Because the serum calcium level in these patients varies from low to high, it is not clear whether every hemodialysis patient with a PTH level > 500 pg/mL is part of a uniform group. The dynamics of PTH secretion in 21 hemodialysis patients with predialysis (basal) intact PTH levels > 500 pg/mL (range, 506 to 1978 pg/mL) has been evaluated. The basal/maximal PTH ratio, an indicator of the degree of relative PTH stimulation in the baseline state, was inversely correlated with the maximal PTH (r = -0.71), the basal serum calcium (r = -0.70), and the difference between the serum calcium at basal and maximal PTH (r = 0.81); the latter is the decrement in serum calcium from baseline necessary to maximally stimulate PTH. Because the basal PTH level appeared to be disproportionately influenced by hypocalcemia, the 21 patients were separated into two groups on the basis of the basal serum calcium (Group I < 9 mg/dL and Group II > 9 mg/dL). Basal PTH was not different between the two groups, even though maximally stimulated PTH (1,219 +/- 204 versus 2,739 +/- 412 pg/mL; P < 0.01) as induced by hypocalcemia and maximally suppressed PTH (217 +/- 37 versus 528 +/- 104; P = 0.05) as induced by hypercalcemia were less in Group I with the low basal calcium; moreover, the ratio of basal/maximal PTH was higher (73 +/- 6 versus 47 +/- 5%; P < 0.01) in Group I with the low basal calcium. These results suggest that the reason for a basal PTH > 500 pg/mL may be different among hemodialysis patients. In hypocalcemic patients, the low serum calcium appeared to be a major impetus for the high basal PTH level. In conclusion, (1) the maximally stimulated PTH appears to provide a better means of separating patients with marked secondary hyperparathyroidism than the basal PTH and (2) hemodialysis patients with basal PTH levels > 500 pg/mL may not be a uniform group.