Fibroblast growth factor 23, but not parathyroid hormone, is associated with urinary phosphate regulation in patients...

Fibroblast growth factor (FGF) 23 plays an important role in regulation of renal phosphate excretion in patients with chronic kidney disease. However, it remains undetermined whether FGF23 is closely linked to renal phosphate handling in patients with low glomerular filtration rate (GFR). The present cross-sectional study included 52 outpatients undergoing peritoneal dialysis with urine volume ≥ 100 mL/day. The primary outcome was level of urinary phosphate excretion, and the secondary outcomes were tubular maximal reabsorption of phosphate normalized to GFR (TmP/GFR), an index of the renal threshold for phosphate excretion, and level of peritoneal phosphate excretion. Variates of interest were serum FGF23 and parathyroid hormone (PTH) levels. The median and interquartile range of serum FGF23 level, TmP/GFR, and total urinary and peritoneal phosphate excretion were 5610 (1493-11 430) ng/mL, 1.30 (0.44-1.86) mg/dL, 117 (40-234) mg/day, and 208 (156-250) mg/day, respectively. Multivariate linear regression analysis revealed that serum FGF23 level was significantly (P < 0.05) associated with TmP/GFR negatively and significantly (P < 0.05) associated with urinary phosphate excretion positively, even after adjusting for confounders. In contrast, none of the three outcome variates was associated with serum PTH level. Neither serum FGF23 nor PTH level was associated with peritoneal phosphate excretion. The present study indicates that FGF23, but not PTH, is involved in urinary phosphate regulation, even in patients on peritoneal dialysis with residual renal function.