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INTRODUCTION: The coronavirus disease 2019 (COVID-19), causing a worldwide pandemic, affects various organs and systems. The effects of COVID-19 on the thyroid axis remain uncertain. Our aim is to examine the changes in thyroid function tests in patients with COVID-19 and to evaluate the effectiveness of these changes in predicting the prognosis. METHODS: This retrospective study was conducted in 1891 adult patients visited to the emergency department with suspected COVID-19. The diagnosis of COVID-19 was confirmed by reverse transcriptase-polymerase chain reaction (rt-PCR), and patients were divided into two groups as those with and without COVID-19. COVID-19 patients were divided into groups according to their clinical severity and prognosis. The groups were compared in terms of free triiodothyronine (FT3), free thyroxine (FT4), and thyrotropin (TSH) levels. RESULTS: The rate of laboratory-confirmed COVID 19 patients was 54.7% in the patients included in the study. In COVID-19 patients, the hospitalization rate was 56.7%, the intensive care unit admission rate was 7.7%, and the in-hospital mortality rate was 5.4%. Most of the COVID-19 patients (60.1%) were euthyroid. Hyperthyroidism (overt 3.7%-1.3%, p=0.001; secondary 13.6–4.6%, p<0.001) and non-thyroid illness syndrome (NTIS) (18.9–3%, p<0.001) were more common in COVID-19 patients than in non-COVID19 patients. Patients with COVID-19 had lower TSH and FT3 levels and higher FT4 levels compared to those without COVID-19. Similarly, among COVID-19 patients, lower TSH and FT3 and higher FT4 levels were seen in deceased patients compared to survivors (p<0.001). DISCUSSION AND CONCLUSION: High FT4 levels combined with low TSH and FT3 levels may represent a type of NTIS that occurs in the acute phase of COVID-19. This pattern appears to be predictor of poor outcome of patients with COVID-19.