Chronic autoimmune thyroiditis (ATD) frequently overlaps with autoimmune rheumatic diseases. The aim of this study was to evaluate the prevalence of various non-organ-specific autoantibodies in patients with ATD, as well as the presence of ATD in patients with systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). Subjects and Methods: Group 1 comprised 80 patients with ATD, and group 2 contained 80 patients with SLE or RA. A control group consisted of 34 healthy subjects. Group 1 was examined for the presence of non-organ-specific autoantibodies. Serum fT3, fT4, TSH, and antibodies against thyroglobulin, thyroperoxidase, as well as ultrasound of thyroid gland, were determined in group 2. Results: Patients with ATD had a significantly higher prevalence of antinuclear antibodies (ANA) than control subjects (45% vs. 14.7%, P < 0.001). There were no significant differences in the prevalence of other antibodies between the groups. ANA-positive patients were younger than ANA-negative ones and had significantly higher anti-TG values (P < 0.05). The prevalence of ATD in group 2 was significantly higher than in the control subjects (24% vs. 8%, P < 0.05). No significant differences in the prevalence of ATD were detected between SLE and RA. Conclusion: The authors conclude that ANA is the most frequent non-organ-specific antibody associated with ATD, while the other antibodies occur rarely. The prevalence of ATD in SLE and RA patients was 24%. These results indicate that it is clinically important to screen patients with SLE and RA for the coexistence of thyroid autoimmune disease.