Background/aimsSelenium (Se), an antioxidant agent, is effective in preventing mild Graves’ orbitopathy (GO) deterioration. However, the significant risk of low serum Se concentration for GO progression has not been identified. Here, we aimed to investigate the risk of relative Se insufficiency and to identify its optimal cut-off value in the development of disease severity in patients with GO.MethodsSerum Se levels were prospectively measured in 100 consecutive patients with GO. The patients were classified into groups with mild and severe GO (logistic regression analysis outcome). A receiver operating characteristic (ROC) curve and the minimum p value corresponding to χ2 statistics were analysed to select the optimal cut-off Se level for the diagnosis of severe orbitopathy.ResultsThirty-two patients (32%) had mild GO and 68 (68%) had severe GO. The ROC revealed a cut-off Se level of 93 µg/L. Se levels ≤93 µg/L were observed in 48.5% and 12.5% of the patients in the severe and mild (p<0.001) groups, respectively. The risk estimate (OR) for an Se level ≤93 µg/L was 8.14 (95% CI 2.39 to 27.75). It remained a significant risk factor after adjusting for age, sex, thyroid status, smoking status, thyroidectomy and radioactive iodine.ConclusionRelative Se insufficiency (≤93 µg/L) is a potential risk factor for severe GO development. An evaluation of Se status is recommended in patients with GO for predicting disease progression and guiding supplementation therapy.