The aim of the present study was to investigate the prevalence of antipituitary antibodies (APA) in patients with autoimmune thyroid disease as determined by Western blot analysis and enzyme-linked immunosorbent assay (ELISA). Results by Western blot analysis showed positivity for APA in serum of 22.4% of patients with Graves' disease (n=143, p<0.05) and 18.5% of patients with Hashimoto's thyroiditis (n=54, p<0.05), which were significantly higher than 6.2% in healthy controls (n=97). Similar results were obtained with ELISA. The titers of APA measured by ELISA (APA/ELISA) remained unchanged before and after therapy with antithyroid drug for Graves' disease, while thyrotropin-binding inhibitor immunoglobulins (TBII) decreased significantly. Similarly, no changes in APA by Western blot analysis were observed after therapy. In patients with Graves' disease, APA were not associated with thyroid status. There was no difference in APA between patients with positive and negative thyroid autoantibodies. A significant but weak positive correlation between APA/ELISA and anti-human GH measured by ELISA (anti-hGH/ELISA) was observed in patients with Graves' disease (r=0.601 p<0.001) and Hashimoto's thyroiditis (r=0.428 p<0.005). These findings have demonstrated the existence of APA detected by Western blot analysis and ELISA in some cases of autoimmune thyroid disease. The present results suggest that hGH and other antigens may be involved in APA in patients with Graves' disease and Hashimoto's thyroiditis.