Supplementary Material for: Parathyroid Hormone, Fibroblast Growth Factor 23, and Parameters of Phosphate Reabsorption

Background: The serum phosphorus concentration ([P]_s) is the sum of E_P/C_cr and TR_P/C_cr, where C_cr is creatinine clearance and E_P and TR_P are rates of excretion and reabsorption of phosphate. In chronic kidney disease (CKD), parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) mediate reduction of TR_P/C_cr, and [PTH] and [FGF23] are linear functions of E_P/C_cr. If controls and patients with CKD are considered together, TR_P/C_cr is a hyperbolic function of E_P/C_cr. Given these observations, we hypothesized that hyperbolas would describe relationships of phosphate reabsorption to [PTH] and [FGF23]. Methods: We studied 30 patients and 28 controls with mean eGFR of 29.5 and 86.0 mL/min/1.73 m², respectively. All analyses combined both subsets. We measured fasting [PTH] 1–84 and intact [FGF23], and determined contemporaneous E_P/C_cr, TR_P/C_cr, fractional excretion of phosphorus (FE_P), and phosphate tubular maximum per volume of filtrate (Tm_P/GFR). We examined linear regressions of TR_P/C_cr and Tm_P/GFR on 100/[PTH] and 100/[FGF23]; from linear equations we derived hyperbolic equations relating reabsorptive parameters to hormone concentrations. Results: TR_P/C_cr and Tm_P/GFR were linear functions of 100/[PTH] and 100/[FGF23] and hyperbolic functions of [PTH] and [FGF23]. TR_P/C_cr and Tm_P/GFR fell minimally over the ranges of E_P/C_cr, [PTH], and [FGF23] seen in CKD. FE_P rose with E_P/C_cr despite stable phosphate reabsorption. Conclusions: Hyperbolas describe relationships of TR_P/C_cr and Tm_P/GFR to [PTH] and [FGF23] if subjects with normal and reduced GFR are analyzed together. Although FE_P rises with [PTH] and [FGF23] as GFR falls, the simultaneous increments do not signify hormonally mediated reductions in phosphate reabsorption.