We aimed to investigate circulating levels of Activin A, Follistatin-Like Protein-1 (FSTL1), and Follistatin-Like Protein-3 (FSTL3) in patients with RA, RA-ILD, idiopathic pulmonary fibrosis (IPF), and healthy controls and explore their associations with disease activity and pulmonary function parameters. The gene discussed is FSTL3; the disease is pulmonary fibrosis.