Evaluation of the Free Triiodothyronine/Free Thyroxine ratio and Euthyroid Sick Syndrome in patients with COVID-19: A cross-sectional study

Aim: In our study, we aimed to evaluate the prevalence of euthyroid sick syndrome among patients with COVID-19, and the relationship between the possible change in free-triiodothyronine/free-thyroxine (fT3/fT4) ratio and the biochemical parameters used during the follow-up of COVID-19. Material and Methods: A total of 114 sequential patients between 18-65 years of age, who were treated in the internal medicine service allocated to patients with COVID-19, were included in the study. fT3, fT4, thyroid-stimulating hormone (TSH), thyroid peroxidase antibodies, and thyroglobulin antibodies antibody levels were analyzed. fT3/fT4 ratio was calculated. Results: According to the results, only isolated low fT3 levels were found in 39.5% (45/114) of the patients, while 25.4% (29/114) had both low TSH and fT3 levels, 31.6% (36/114) had autoimmune thyroid disease, and 3.5% (4/114) had normal results in the thyroid function tests. The fT3/fT4 ratio was also determined to be low in all groups. However, no significant correlation was found between the low fT3/fT4 ratio and the parameters used in monitoring COVID-19. Discussion: We believe that studies with a wider scope of participants are needed for testing the reduction in fT3/fT4 ratio with other biochemical parameters in patients with COVID-19 and without known thyroid disease.