Introduction: Selenium supplementation has been suggested as a therapeutic option for Hashimoto's thyroiditis, but its effects on thyroid function and other patient-related outcomes remain unclear.
Methods: This retrospective cohort study utilised a global federated health research network to compare patients with Hashimoto's thyroiditis who received selenium supplementation after diagnosis to those who did not. Propensity score matching was performed to balance baseline characteristics. Primary outcomes included longitudinal thyroid function measures, while secondary outcomes included development of autoimmune diseases, thyroid cancer, need for thyroidectomy and all-cause mortality. All outcomes were followed for 5 years.
Results: After matching, 2347 patients were included in each cohort. Selenium supplementation was associated with persistently higher mean TSH, free T4 and TPOAb levels, as well as greater proportions of elevated TPOAb (> 35 IU/mL) at 3 years (6.9% vs. 5.0%, p = 0.01) and 5 years (7.9% vs. 5.3%, p < 0.01). The incidence of autoimmune disorders was higher in the selenium group (RR 1.33, 95% CI 1.14-1.56), particularly Sjögren's syndrome (RR 1.61, 95% CI 1.03-2.52) and psoriasis (RR 2.69, 95% CI 1.52-4.76). The rates of thyroid cancer and need for thyroidectomy were similar between groups. However, selenium use was associated with increased all-cause mortality (RR 1.38, 95% CI 1.09-2.03).
Conclusion: Selenium supplementation in Hashimoto's thyroiditis was associated with worsened biochemical autoimmunity, increased autoimmune comorbidity and higher mortality. These findings should be interpreted cautiously given the absence of baseline selenium measurements, which limits assessment of deficiency status. Nevertheless, they highlight the need for further research to confirm these results.
Keywords: Hashimoto's thyroiditis; TriNetX; autoimmune; propensity score matching; selenium.
© 2026 The Author(s). Endocrinology, Diabetes & Metabolism published by John Wiley & Sons Ltd.