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 m 2 , 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.