Recent studies have demonstrated that T-helper 17 lymphocytes (Th17), which produce mostly IL-17, play a major role in several autoimmune diseases commonly thought to be Th1-related, including Hashimoto's thyroiditis (HT). IL-23, a member of the IL-12 cytokine family, is known to guide T cells toward the Th17 phenotype and its serum levels are increased in several autoimmune disease. Few data are available in the literature on IL-23 in HT. Using IL-23 Quantikine ELISA Kit (lower limit of detection 2.7 pg/mL) we analyzed the serum levels of IL-23 in 81 HT patients (75 females and 6 males, aged 14-70; mean age 39±17 years), and an age- and sex-matched group of 80 healthy persons. Both patients and controls did not receive any treatment. The positive detection rates of serum IL-23 were significantly higher in patients with HT: 56% of HT patients had detectable IL-23 in serum compared to 36% of healthy subjects (Chi χ² test, p=0.014). Moreover, HT patients had significantly higher serum concentrations of IL-23 (157.38 ± 17.92 pg/mL) in comparison with healthy controls (21.46 ± 5.4 pg/mL; p <0.0001). No significant correlation was found between serum levels of IL-23 and Tg-Ab or TPO-Ab levels, as well as with TSH values, in HT patients. In conclusion, serum IL-23 is increased in euthyroid and untreated HT patients, as compared to healthy subjects. Our data suggest that IL-23 would play a role in the pathogenesis of HT.