GDF15 and idiopathic pulmonary fibrosis: GDF-15 serum levels were significantly negatively correlated with leptin in AE-IPF subjects (r = - 0.329, p = 0.024) (Supplementary Figure 1B) but not in S-IPF or HC cases (r = 0.180, p = 0.168 and r = - 0.345, p = 0.062, respectively) (Supplementary Figures 1C–D).