Interleukins play an important role in the development of autoimmune disorders. The aim of this study was to compare the concentration of interleukin 29 (IL-29) between healthy controls (CS) and patients with selected thyroid disorders: Graves' disease (GD), Hashimoto's thyroiditis (HT), and subacute thyroiditis (SAT).The following parameters were examined in the group of 95 individuals (45 with GD, 22 with HT, 28 with SAT) and 72 CS: thyroid hormones and autoantibodies, inflammatory markers and the concentration of IL-29 in serum.The concentration of IL-29 in the GD subgroup was higher than that in the CS subgroup [264.0 (62.5-1018.0) vs. 62.5 (62.5-217.0) pg/mL, p=0.001]. We found no differences in IL-29 concentrations between the CS and HT or SAT subgroups. Multivariable linear regression analysis indicated that IL-29 was a statistically significant independent predictor of GD presence (r = 0.24; p=0.003) after adjustment for TRAb (R2 =0.45; p<0.001). The ROC analysis of IL-29 at GD diagnosis revealed an IL-29 cutoff of 123 pg/mL (sensitivity 0.689 and specificity 0.625) as the best value, which significantly indicated the presence of GD [area under the ROC curve (AUC): 0.676; 95% confidence interval (CI): 0.574-0.778, p<0.001].This is the first study to demonstrate elevated IL-29 serum levels in patients with GD. Our results suggest that IL-29 might be engaged in one of the pathogenetic pathways of GD, but no HT and SAT. Future studies are required to evaluate the potential of the protein as a therapeutic target in GD.