GPX4 and chronic obstructive pulmonary disease: The results of multivariate Cox proportional risk analysis revealed that low serum levels of GPX4 (HR 3.753; 95% CI 1.339–10.521; p = 0.012), high serum levels of sTfR1 (HR 2.970; 95% CI 1.247–7.071; p = 0.014), and high sTfR1/GPX4 (HR 3.668; 95% CI 1.380–9.750; p = 0.009), as well as absolute serum levels of sTfR1 (HR 1.000; 95% CI 1.000–1.000; p = 0.049) and sTfR1/GPX4 (HR 1.00; 95%IC 1.000–1.000; p = 0.024) were potential risk factors for COPD-related hospitalization (Fig. 2; Table 4).