OBJECTIVE Interleukin-33 (IL-33), a 30 kDa cytokine, is a member of IL-1 family. It is considered to be an autoimmune biomarker associated with T helper 2 (Th 2) response. γ-interferon is also produced by T helper 1 (Th 1) cells to induce cellular responses. γ-interferon is a 143-amino acid residue glycoprotein with several biological functions including potent anti-viral activity, stimulation of macrophage activity, modulation of Major Histocompatibilty Complex class I/class II expression, and regulation of a diversity of specific immune responses. The aim of this study was to investigate the serum levels of IL-33 and γ-interferon in different thyroid disorders. METHODS Twenty patients with Graves' disease, 21 patients with Hashimoto hypothyroidism, 21 euthyroid Hashimoto patients, and 27 control subjects were recruited to this study. Blood samples were drawn and IL-33 and γ-interferon tests were analyzed from 89 participants. Serum IL-33 and γ-interferon analyses were performed by enzyme-linked immunosorbent assay. RESULTS There was no statistically significant difference between groups for serum γ-interferon levels. Serum IL-33 concentrations were significantly higher in Graves' disease group compared to the other groups (p<0.000) There was a positive correlation between serum IL-33 and free triiodothyronine (fT3) and thyroxine (fT4). Also, negative correlation between serum IL-33 and thyroid-stimulating hormone (TSH) was statistically significant (p<0.000). CONCLUSIONS The correlation of serum IL-33 with thyroid hormone levels may be a useful indicator for Graves' disease. These findings may help to make evident the pathophysiologic processes of the autoimmune thyroid diseases and improve therapeutic methods. .