Background: Since the outbreak of the coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in December 2019, it has affected >200 countries, areas, or territories in 6 continents. At present, whether COVID-19 has an effect on thyroid function is unclear. The aim of this study was to evaluate thyroid function in patients with COVID-19. Methods: Clinical manifestations, laboratory results, and chest computed tomography scans were retrospectively reviewed for 50 patients with laboratory-confirmed COVID-19 without a history of thyroid disease who underwent thyroid function testing during their course of COVID-19 infection and after recovery. They were admitted to the First Affiliated Hospital, College of Medicine, Zhejiang University, Hangzhou, China, between January and March 2020. Healthy participants who underwent routine physical checkups and non-COVID-19 pneumonia patients with a similar degree of severity during the same period were included in the study as the control group. Thyroid hormone and thyrotropin (TSH) levels were analyzed and compared between the COVID-19 and control groups. Results: TSH lower than the normal range was present in 56% (28/50) of the patients with COVID-19. The levels of TSH and serum total triiodothyronine (TT3) of the patients with COVID-19 were significantly lower than those of the healthy control group and non-COVID-19 pneumonia patients. The more severe the COVID-19, the lower the TSH and TT3 levels were, with statistical significance (p < 0.001). The degree of the decreases in TSH and TT3 levels was positively correlated with the severity of the disease. The total thyroxine (TT4) level of the patients with COVID-19 was not significantly different from the control group. All the patients did not receive thyroid hormone replacement therapy. After recovery, no significant differences in TSH, TT3, TT4, free triiodothyronine (fT3), and free thyroxine (fT4) levels were found between the COVID-19 and control groups. Conclusions: The changes in serum TSH and TT3 levels may be important manifestations of the courses of COVID-19.

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