Relationships between rheumatoid arthritis (RA) and the thyroid gland have been studied extensively for a long time. The studies of this problem have focused mainly on: (a) the functional and immune thyroid gland abnormalities in patients with previous history of RA, and (b) joint changes in patients with previous autoimmune thyroid diseases. Thyroid dysfunctions in RA patients are most often of autoimmune nature; they are accompanied by elevated thyroid autoantibody titers. The RA patients usually present with eu-, hypo- or hyperthyroid manifestations. The concurrent affection of joints and thyroid gland is related most probably to a genetic predisposition determined by the affiliation to a certain HLA type, most often HLA-DR. Joint abnormalities in thyroid gland disorders may be of different character (generally polyarthritis) and they are due to hypothyroidism. One possible explanation of the presence of two or more autoimmune diseases in one individual is microchimerism - the presence of a small number of fetal cells in the mother as well as maternal cells in the fetus. These data provide grounds for tests to be performed in all cases of RA so that thyroid autoantibodies and thyroid dysfunctions can be detected early and treated adequately.