Autoimmune thyroid disease (AITD), known as the most common organ-specific autoimmune disorder, is frequently accompanied by other organ and non-organ-specific autoimmune diseases, including rheumatoid arthritis (RA). Although the exact pathogenic mechanisms of the coexistence of autoimmune disorders are still not completely defined, genetics, immune defects, hormones and environmental factors may play key roles in polyautoimmunity. In this review, the prevalence of AITD and antithyroid autoantibodies in RA patients and rheumatic manifestations in association with thyroid autoimmunity are discussed. Finally, we review the role of genetics in the association of both AITD and RA, especially CTLA-4 and PTPN22 polymorphisms.