Parathyroid suppression by intraperitoneal calcitrol (1,25(OH)2D3) during peritoneal dialysis. The purpose of this study was to determine if parathyroid hormone (PTH) suppression could be achieved by increasing calcium mass transfer (Ca MT) with high dialysate Ca (4 mEq/liter) or via intraperitoneal (i.p.) 1,25(OH)2D3 in patients undergoing continuous ambulatory peritoneal dialysis. Eleven patients were dialyzed for two months with standard Ca dialysate (3.5 mEq/liter) followed by two months with 4.0 mEq/liter Ca, then by three months of i.p. 1,25(OH)2D3. During the latter period, patients were randomized to groups whose dialysate contained either 3.5 mEq/liter or 4.0 mEq/liter Ca. We found that 4.0 mEq/liter Ca dialysate more than doubled Ca MT (37 ± 17 mg/day to 84 ± 6 mg/day) leading to a modest fall (P<0.05) in PTH levels (84 ± 5.5% of controls). Ionized calcium levels did not change. With i.p. 1,25(OH)2D3, however, ionized calcium rose significantly (P<0.001) leading to a decline in PTH levels to 53.9 ± 7.9% of control values. Serum 1,25(OH)2D3 levels rose from undetectable to 47.7 ± 7.2 pg/dl (normal range 20 to 35). These studies indicate that increasing Ca MT using a 4.0 mEq/liter Ca dialysate leads to a small reduction in PTH concentrations. On the other hand, i.p. 1,25(OH)2D3 is well absorbed into the systemic circulation, raises ionized calcium levels, and leads to a marked suppression of PTH. Thus, i.p. 1,25(OH)2D3 may be a simple and effective means to suppress secondary hyperparathyroidism in patients undergoing CAPD.