Serum phosphate and phosphate-regulatory hormones in COPD patients

Fibroblast growth factor 23 (FGF23) regulates phosphate metabolism by increasing renal phosphate excretion and decreasing 1.25-dihydroxyvitamin D synthesis. Reports about hypophosphatemia in patients with chronic obstructive pulmonary disease (COPD) suggest altered phosphate metabolism. Therefore, we hypothesized that disturbances in phosphate-regulatory hormones such as FGF23 and parathyroid hormone (PTH) are present in COPD patients.

We investigated 40 COPD patients (63.5 ± 9.9 years, 27 male), each matched with two age- and sex-matched controls without any primary lung disease. COPD patients underwent lung function testing in advance. All patients had a glomerular filtration rate (GFR) > 60 mL/min/1.73m². We measured concentrations of intact FGF23 (iFGF23) and c-terminal FGF23 (c-term FGF23), phosphate, parathyroid hormone (PTH) and C-reactive protein (CRP) levels in COPD patients and controls.

Phosphate (1.0 ± 02 vs. 1.1 ± 0.2 mmol/L; p = 0.027), PTH (54.2 ± 29.4 vs. 68.7 ± 31.8 pg/mL; p = 0.002) and iFGF23 (46.3 ± 29.0 vs. 57.5 ± 33.5 pg/mL; p = 0.026 ) levels were significantly lower in COPD patients compared with controls. There was a significant negative correlation between c-term FGF23 and total lung capacity (r = - 0.4; p = 0.01), and between c-term FGF23 and CRP in COPD patients (r = 0.48; p = 0.002). iFGF23 and c-term FGF23 were positively correlated with phosphate and PTH in the control group.

We confirmed lower average serum phosphate levels in COPD patients compared with controls. However, our data do not suggest a causative role for FGF23 or PTH in COPD because levels of both phosphate-lowering hormones appear to be adaptively decreased as well. Therefore, further investigations are needed to identify the pathogenesis of low phosphate levels in patients with COPD and the relationship between phosphate-regulatory hormones and disease progression.