To verify the greater prevalence of circulating thyroid hormone autoantibodies (THAb) in primary Sjogren syndrome (SS) vs Hashimoto's thyroiditis (HT) and Graves' disease (GD), we measured THAb in the serum of patients with these 3 diseases who were sampled from 1998-1999 (no.=20, 88, 25) and 1990-1992 (no.=13, 75, 31). Patients with rheumatoid arthritis (RA) (no.=23 and 16) and other collagenoses (no.=20 and 16) were also studied. A third series of patients with these 5 diseases was studied from 1975-1982, and data have been taken into account. THAb were detected using a specific radioimmunoprecipitation method, and their presence was correlated with the presence of TG antibodies (TGAb). We found that IgG antibodies against T3, T4 or both were present with these prevalences in the 1975-1982, 1990-1992 and 1998-1999 series: HT=1, 4, 20%; GD=2, 6, 32%; SS=20, 31, 50%; RA=0, 12, 26%; other collagenoses=0, 0, 0%. The majority of the Sjogren or arthritis cases positive for THAb were negative for TGAb, while the opposite was true for the 2 autoimmune thyroid diseases. We conclude that prevalence of THAb in the 2 non-thyroid autoimmune diseases is greater than in the 2 thyroid autoimmune diseases. In addition, prevalence of THAb is increasing over time regardless of disease. Molecular similarity between extra-thyroid connective proteins (specifically associated to primary SS and RA) and iodinated regions of TG, and an increased preponderance of environmental factors as triggers of autoimmune diseases might account for our findings.