Abstract
Background SARS-CoV-2 infects cells via angiotensin-converting enzyme 2 (ACE2). ACE2 levels are high in the thyroid gland. Although the thyroid gland can be directly infected in COVID-19 patients, the hypothalamic-pituitary-thyroid axis is also affected. Therefore, changes in thyroid function occur in COVID-19 patients. This study aimed to examine the effect of thyroid function tests on the prognosis of COVID-19. Methodology A total of 146 patients who were diagnosed with COVID-19 and treated in the intensive care unit between August and November 2021 and who had no previous history of thyroid disease were included in the study. Demographic information, laboratory tests, and thyroid hormone levels during hospitalization and discharge patterns were evaluated. The patients were divided into two groups: group I included those who were discharged after recovery, and group II included those who did not respond to treatment and died. Results When the fT3 and fT4 levels of the patients were compared, the hormone levels decreased as the clinical severity of the disease increased. The amount of decrease in hormone levels was mostly seen in group II. In the recovered patient group, the amount of hormone decreased was less. The difference between fT3 and ft4 values between the groups was found to be statistically significant (P = 0.015 and P = 0.004). In addition, the difference between the groups' C-reactive protein (CRP), D-dimer, and ferritin values was statistically significant (P = 0.036, P = 0.022, and P < 0.000, respectively). There was no statistically significant difference between the groups in terms of demographic characteristics (P > 0.05). Conclusions Thyroid hormone changes were found to be an important prognostic parameter affecting disease severity and mortality in COVID-19 patients and can be used to predict mortality.