FGF21 expression levels in atrial tissue and serum FGF21 levels are significantly increased in atrial fibrillation patients with rheumatic heart disease, indicating that FGF21 is involved in the development and maintenance of atrial fibrosis in atrial fibrillation with rheumatic heart disease in a paracrine manner (Wang R. et al., 2015). Here, FGF21 is linked to rheumatic heart disease.