PTH response to the predialysis serum calcium concentration in hemodialysis patients with severe (A), moderate (B), and mild (C) hyperparathyroidism. PTH response to the predialysis serum calcium concentration in hemodialysis patients with severe (A), moderate (B), and mild (C) hyperparathyroidism. In hemodialysis patients with severe hyperparathyroidism (A; nonresponders to calcitriol treatment shown in Table 2), both maximal and basal PTH increased as the iCa concentration increased, whereas the basal/maximal PTH ratio did not change. In hemodialysis patients with moderate hyperparathyroidism (B; responders to calcitriol treatment shown in Table 2), the maximal PTH value increased as the iCa concentration increased, but there was no correlation between basal PTH and iCa; however, an inverse correlation was present between the basal/maximal PTH ratio and iCa, suggesting that an increasing serum calcium concentration decreased basal PTH levels. In patients with mild hyperparathyroidism (C), all of whom had diabetes and needed a basal PTH value <300 pg/ml for inclusion in the study, an inverse correlation was present between both basal PTH and the basal/maximal PTH ratio and the serum calcium concentration. It should also be noted that the y axis scale for basal and maximal PTH is less than in A and B. Data are means ± SEM. A and B are reprinted from reference (70), with permission from Kidney International; C was produced from data available in reference (129). Arnold J. Felsenfeld et al. CJASN 2007;2:1283-1305 ©2007 by American Society of Nephrology