15 patients with rheumatoid arthritis (14 women and 1 man aged between 30 and 75 years) were compared to a control group of 12 women and 1 man (aged between 22 and 77 years) admitted to the rheumatology unit at the same time for benign diseases. Both groups were examined for: the presence of a goitre, family history and/or clinical hormonal dysfunction, the plasma levels of total cholesterol, free thyroxin (FT4), free triiodothyronine (FT3), before and 2 hours after 50 micrograms of thyrotropin IV, thyrostimulin half an hour before and one hour after thyrotropin and the levels of the antimicrosomal anti-thyroid antibodies and the anti-thyroglobulin antibodies. None of the patients with rheumatoid arthritis had any clinical hormonal dysfunction. However, 6 of the 15 patients presented a homogeneous goitre, 5 of these 6 patients had a family history of goitre and 2 had positive antibodies. In comparison with the control group, the 15 cases of rheumatoid arthritis had a significant decrease (m +/- sd) in the FT4 (13.20 +/- 2.50 pmol/l vs 15.60 +/- 2.47; p less than 0.02), FT3 (3.80 +/- 1.12 pmol/l vs 5.50 +/- 0.93; p less than 0.001) and a rise in the T3 with low thyrotropin (5.70 +/- 1.60 vs 8.50 +/- 1.50; p less than 0.001), while the levels of thyrostimulin and the thyrostimulin peak under thyrotropin were not modified.(ABSTRACT TRUNCATED AT 250 WORDS)