AbstractBackgroundAutoimmune thyroid disease (AITD) mainly includes Graves’ disease (GD) and Hashimoto's thyroiditis (HT), which is caused by individual genetics, autoimmune dysfunction, and a variety of external environmental factors. Interleukin (IL)‐38 is involved in a wide range of autoimmune diseases, but little is known about IL‐38 expression in AITD.MethodsFifty patients with GD, 50 with HT, and 50 healthy controls (HC) were enrolled in this study. Basic information of the participants was obtained through a physical examination. Immunological data were obtained by an automatic chemiluminescence immunoanalyzer. C‐reactive protein (CRP) concentrations and the white blood cell count were measured. Serum IL‐38 levels were determined by an enzyme‐linked immunosorbent assay.ResultsSerum IL‐38 levels were significantly lower in the GD and HT groups than in the HC group (bothp < 0.01). Serum CRP concentrations were significantly lower in the HT group than in the HC group (p < 0.05). Receiver operating characteristic curve analysis showed that the area under the curve was 0.7736 (p < 0.01) for IL‐38 and 0.7972 (p < 0.01) for IL‐38 combined with CRP in the GD group. In the HT group, the area under the curve was 0.7276 (p < 0.01) for IL‐38 and 0.7300 for IL‐38 combined with CRP (p < 0.01).ConclusionsThe results suggest that serum IL‐38 level is a potential new diagnostic biomarker in patients with GD and HT.