GPX4 and chronic obstructive pulmonary disease: The results of multivariate Cox proportional risk analysis identified low serum levels of GPX4 (HR 2.301; 95% CI; 1.426–3.713; p = 0.001), high serum levels of sTfR1 (HR 1.850; 95% CI 1.212–2.823; p = 0.004), and high sTfR1/GPX4 (HR 2.223; 95% CI 1.441–3.428; p < 0.001), as well as absolute serum levels of GPX4 (HR 1.000; 95% CI 1.000–1.000; p = 0.049) and sTfR1 (HR 1.000; 95% CI 1.000–1.001; p = 0.001), and sTfR1/GPX4 (HR 1.00; 95% CI 1.000–1.000; p = 0.001) as potential independent risk factors for moderate COPD exacerbation (Fig. 1; Table 4).