The authors made in patients with confirmed or classical rheumatoid arthritis, using RIA, functional tests of the thyroid gland, i.e. tetra-, triiodothyronine and thyroid stimulating hormone. While deviations of tetraiodothyronine levels from normal very rare, a reduction of triiodothyronine was recorded in 50% and of TSH in 27.2%. Clinically manifest thyropathy, i.e. hyperthyroidism was found only in one female patient. The authors investigated changes in the values of the above tests in the course of rheumatoid arthritis and a dysbalance in the regulation of thyroid functions. The investigation comprises also a group of 18 subjects with rheumatoid arthritis where antibodies against the thyroid gland were assessed. The latter were elevated in two patients.