Hyperphosphatemia with elevated serum PTH and FGF23, reduced 1,25(OH)₂D and normal FGF7 concentrations characterize...

Hyperphosphatemia confers adverse cardiovascular outcomes, and commonly occurs in late-stage CKD. Fibroblast growth factor 7 (FGF7) is a phosphaturic peptide which decreases renal phosphate transport in vitro and in vivo. Serum FGF7 concentrations are reduced in hyperphosphatemic patients with hypophosphatasia and are elevated in some hypophosphatemic patients with tumor-induced osteomalacia. No data, however, are available on whether circulating FGF7 concentrations increase to compensate for phosphate retention in CKD patients.

This was a cross-sectional study performed among 85 adult patients with varying estimated glomerular filtration rates (eGFR). We measured serum intact FGF7 (iFGF7) concentration using an iFGF7 immunoassay and determined its associated factors. Relationships between eGFR and mineral metabolism biomarkers [phosphate, iFGF7, iFGF23, parathyroid hormone (PTH), and 1,25-dihydroxyvitamin D (1,25(OH)₂D)] were explored.

For eGFRs of ≥ 60 (n = 31), 45-59 (n = 16), 30-44 (n = 11), 15-29 (n = 15), and < 15 mL/min/1.73 m² (n = 12), median (IQ25-75) iFGF7 concentrations were 46.1 (39.2-56.9), 43.1 (39.0-51.5), 47.3 (38.3-66.5), 47.7 (37.7-55.8), and 49.6 (42.5-65.6) pg/mL, respectively (P = 0.62). Significant increases in serum iFGF23, PTH, and phosphate were observed at eGFRs of < 33 (95 % CI, 26.40-40.05), < 29 (95 % CI, 22.51-35.36), and < 22 mL/min/1.73 m² (95 % CI, 19.25-25.51), respectively, while significant decreases in serum 1,25(OH)₂D were observed at an eGFR of < 52 mL/min/1.73 m² (95 % CI, 42.57-61.43). No significant correlation was found between serum iFGF7 and phosphate, iFGF23, PTH or 1,25(OH)₂D. In multivariable analyses, body mass index (per 5 kg/m² increase) was independently associated with the highest quartile of serum iFGF7 concentration (OR, 1.20; 95 % CI, 1.12-1.55).

Compensatory decreases in circulating 1,25(OH)₂D and increases in circulating iFGF23 and PTH, but not iFGF7, facilitate normalization of serum phosphate concentration in early stages of CKD. Whether other circulating phosphaturic peptides change in response to phosphate retention in CKD patients deserves further study.