The disrupted autoimmune response in Hashimoto's thyroiditis (HT) has long been considered to be dominantly T helper type 1 (Th1) mediated. Recent advances in the field of immunology have introduced a new class of effector T cells, named 'Th17', which plays important roles in autoimmune disorders once thought to be merely Th1 mediated. We aimed to examine the levels of major Th17 cytokines in patients with HT in this study. We studied serum interleukin 17 (1L-17) and interleukin 23 (IL-23) levels in 46 newly diagnosed, untreated patients with HT (40 women and 6 men, aged 40.0 +/- 11.8 years) divided into euthyroid (n = 22) and hypothyroid (n = 24) groups and compared them with age and sex matched 26 healthy euthyroid controls without HT (21 women and 5 men; aged 36.0 +/- 12.9 years). Serum IL-17 and IL-23 levels were significantly different among euthyroid and hypothyroid HT patients and controls, with highest levels obtained in the euthyroid HT group (p = 0.041 for IL-17 and p < 0.001 for IL-23). TSH was negatively and FT4 was positively correlated with IL-17 (p = 0.016 for TSH and p = 0.004 for FT4) and IL-23 (p < 0.001 for TSH and p = 0.003 for FT4) levels. There were no correlations between thyroid volumes calculated on thyroid ultrasonography and IL-17 (p = 0.630) or IL-23 (p = 0.321) levels. In conclusion, the levels of IL-17, one of the major effector cytokines of the Th17 system, and IL-23, which had been implicated in the generation, survival and expansion of Th17 cells, are altered in HT. How thyroid hormone status and the course of disease affect Th17 system in chronic autoimmune thyroiditis needs to be determined with further studies. (C) 2016 Elsevier Ltd. All rights reserved.