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

2018-05-18T08:35:56Z (GMT) by Phelps K.R. Mason D.L.
<b><i>Background:</i></b> The serum phosphorus concentration ([P]<sub>s</sub>) is the sum of E<sub>P</sub>/C<sub>cr</sub> and TR<sub>P</sub>/C<sub>cr</sub>, where C<sub>cr</sub> is creatinine clearance and E<sub>P</sub> and TR<sub>P</sub> 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<sub>P</sub>/C<sub>cr</sub>, and [PTH] and [FGF23] are linear functions of E<sub>P</sub>/C<sub>cr</sub>. If controls and patients with CKD are considered together, TR<sub>P</sub>/C<sub>cr</sub> is a hyperbolic function of E<sub>P</sub>/C<sub>cr</sub>. Given these observations, we hypothesized that hyperbolas would describe relationships of phosphate reabsorption to [PTH] and [FGF23]. <b><i>Methods:</i></b> We studied 30 patients and 28 controls with mean eGFR of 29.5 and 86.0 mL/min/1.73 m<sup>2</sup>, respectively. All analyses combined both subsets. We measured fasting [PTH] 1–84 and intact [FGF23], and determined contemporaneous E<sub>P</sub>/C<sub>cr</sub>, TR<sub>P</sub>/C<sub>cr</sub>, fractional excretion of phosphorus (FE<sub>P</sub>), and phosphate tubular maximum per volume of filtrate (Tm<sub>P</sub>/GFR). We examined linear regressions of TR<sub>P</sub>/C<sub>cr</sub> and Tm<sub>P</sub>/GFR on 100/[PTH] and 100/[FGF23]; from linear equations we derived hyperbolic equations relating reabsorptive parameters to hormone concentrations. <b><i>Results:</i></b> TR<sub>P</sub>/C<sub>cr</sub> and Tm<sub>P</sub>/GFR were linear functions of 100/[PTH] and 100/[FGF23] and hyperbolic functions of [PTH] and [FGF23]. TR<sub>P</sub>/C<sub>cr</sub> and Tm<sub>P</sub>/GFR fell minimally over the ranges of E<sub>P</sub>/C<sub>cr</sub>, [PTH], and [FGF23] seen in CKD. FE<sub>P</sub> rose with E<sub>P</sub>/C<sub>cr</sub> despite stable phosphate reabsorption. <b><i>Conclusions:</i></b> Hyperbolas describe relationships of TR<sub>P</sub>/C<sub>cr</sub> and Tm<sub>P</sub>/GFR to [PTH] and [FGF23] if subjects with normal and reduced GFR are analyzed together. Although FE<sub>P</sub> rises with [PTH] and [FGF23] as GFR falls, the simultaneous increments do not signify hormonally mediated reductions in phosphate reabsorption.