Background. Although the role of cytokines in the development of Hashimoto's thyroiditis has already been established, its pathogenesis has not yet been clearly elucidated. The aim of our study was to investigate serum transforming growth factor-beta 1 (TGF-beta 1) levels in patients with Hashimoto's thyroiditis as well as the effect of achieving euthyroidism by levothyroxine replacement on TGF-beta 1 levels.,Methods. Twenty nine female, newly diagnosed hypothyroid Hashimoto's thyroiditis patients (16 overt, 13 subclinical hypothyroid) and 25 age- and sex-matched healthy controls were enrolled in the study.,Results. Serum TGF-beta 1 levels were lower in the Hashimoto's thyroiditis group when compared with control cases. Although significant differences were noted in lipid levels and in anthropometric measurements following levothyroxine replacement, serum TGF-beta 1 levels remained unchanged.,Conclusions. Our data suggest that altered TGF-beta 1 levels are associated with the presence of Hashimoto's thyroiditis, not with the treatment of thyroid dysfunction. Autoimmunity may have been triggered as a result of decreased immunosuppressive effect induced by depressed TGF-beta 1 levels in patients with Hashimoto's thyroiditis. (C) 2008 IMSS. Published by Elsevier Inc.