GDF15 and IgG4-related retroperitoneal fibrosis: The presence of retroperitoneal fibrosis (odds ratio (OR) 3.47, 95% confidence interval (CI) 1.2–11.4, P = 0.026) and the presence of parotid gland involvement (OR 3.92, 95% CI 1.0–19.5, P = 0.048) were independently associated with high GDF-15.