Background <p>Subacute thyroiditis (SAT) is an inflammatory disease that induces thyrotoxicosis. Selenium is an essential trace element in thyroid physiology, which has anti-inflammatory and antioxidant properties. However, the relationship between serum selenium levels and SAT has not been well studied. The objective of this study was to evaluate serum selenium levels in patients with SAT compared to healthy controls and to investigate potential correlations between selenium status and clinical outcomes, including disease severity, delayed remission, recurrence, and the development of permanent hypothyroidism.</p> Methods <p>This case-control study included 59 patients with SAT and 50 healthy control subjects. Serum selenium levels were analysed using inductively coupled plasma mass spectrometry.</p> Results <p>The serum selenium levels of patients with SAT were 69.10 (24.60–130.20) µg/L, while those of the control group were 64.20 (39.21–106.80) µg/L (<i>p</i> = 0.121). A negative correlation was detected between serum selenium levels and erythrocyte sedimentation rate, C-reactive protein, neutrophil-to-lymphocyte ratio, free thyroxine, and pain severity. Serum selenium levels did not significantly differ in terms of the response to initial treatment, recurrence, and permanent hypothyroidism.</p> Conclusion <p>The study results showed no significant difference in serum selenium levels between patients with SAT and the control group. These results suggest that although lower serum selenium levels may be associated with a more severe and painful SAT course, there is no impact on the long-term prognosis.</p>