IL-21 levels are elevated in peripheral blood and tissues of patients with various autoimmune diseases, including rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), immune thrombocytopenic purpura (ITP), type 1 diabetes mellitus (DM 1), autoimmune thyroid disease (AITD), primary Sjogren’s syndrome (pSS), and psoriasis [51]. Here, IL21 is linked to autoimmune thyroid disease.