Secreted glycoprotein that is involved in various physiological processes, such as angiogenesis, regulation of the immune response, cell proliferation and differentiation (PubMed:29212066, PubMed:22265692). Plays a role in the development of the central nervous system, skeletal system, lungs, and ureter (By similarity). Promotes endothelial cell survival, migration and differentiation into network structures in an AKT-dependent manner. Also promotes survival of cardiac myocytes (By similarity). Initiates various signaling cascades by activating different receptors on the cell surface such as DIP2A, TLR4 or BMP receptors (PubMed:20054002, PubMed:22265692). Homodimer (By similarity). Interacts with SCN10A (By similarity). Interacts with DIP2A; DIP2A may act as a cell surface receptor for FSTL1 (PubMed:20860622, PubMed:20054002). Interacts with BMP4 (PubMed:29212066). Interacts with CD14; this interaction promotes TL4-mediated signaling cascade (PubMed:22265692). Overexpressed in synovial tissues from rheumatoid arthritis (PubMed:15638044).