Background and Objectives: Euthyroid sick syndrome, or non-thyroidal illness syndrome, has been observed in severely ill patients and has been found to be an index of prognosis. It has been detected in patients with severe infectious diseases, e.g., those with severe COVID-19 infection. Prognostic indicators of the outcome of severe COVID-19 disease are important for the prognosis of individual as well as groups of patients. The aim of this study was to identify euthyroid sick syndrome in patients admitted for severe COVID-19 disease and its relationship to disease severity and outcome. Materials and Methods: In a cohort of patients admitted to hospital for severe COVID-19 disease, thyroid function in patients requiring hospitalization was evaluated by measuring TSH, FreeT3 (FT3), and FreeT4 (FT4) levels. Patients were classified into four groups: a group with uncompromised respiratory function (pO2 > 70 mmHg, without need of oxygen supplementation) (disease severity 1); a group with mild respiratory insufficiency (pO2 50-60 mmHg, in need of oxygen supplementation with nasal cannula) (disease severity 2); a group with severe respiratory insufficiency (pO2 < 50 mmHg, in need of oxygen supplementation with high flow oxygen) (disease severity 3); and a group with severe respiratory insufficiency requiring intubation (pO2 < 60 mmHg on high flow oxygen supplementation) (disease severity 4). Results: In this cohort, euthyroid sick syndrome was diagnosed in 57.1% of the patients. The presence of euthyroid sick syndrome was related to increased disease severity and adverse disease outcome, i.e., death. FT3 levels were inversely related to CRP levels. Conclusions: Euthyroid sick syndrome may be observed in severe COVID-19 disease and is related to increased disease severity and adverse outcomes. Measurement of thyroid hormones in patients hospitalized for severe COVID-19 infection may aid in the prognosis of the disease.