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Purpose: An increasing number of cases of subacute thyroiditis (SAT) related to the coronavirus disease 2019 (COVID-19) and its vaccines continue to be published. The aim of this study was to investigate any change in the incidence and characteristics of SAT by comparing the pre-pandemic and pandemic periods. Methods: This retrospective, single-center study included 432 newly-diagnosed SAT patients between January 2018 and December 2021. The annual frequency of SAT was calculated as the number of newly-diagnosed SAT cases divided by the total number of outpatients that year. Results: The frequencies of newly-diagnosed SAT were 0.136% in 2018, 0.127% in 2019, 0.157% in 2020, and 0.114% in 2021 (p= 0.19). While SAT patients were clustered in the autumn (35.1%) in 2018 and 2019, it was found that this cluster shifted to the winter (33.0%) in 2020 and 2021, in parallel with COVID-19 case peaks (p= 0.017). The patients were separated into two groups as pre-COVID-19 pandemic SAT (n= 272) and COVID-19 pandemic SAT (n= 160). The mean ages of the groups were similar. There were more male patients in the COVID-19 pandemic SAT group than in the pre-pandemic group (30.6% vs. 18.7%, p= 0.005). Frequencies of overt hyperthyroidism and median free-thyroxine levels were significantly higher in the COVID-19 pandemic SAT group (p= 0.029, p= 0.001). Treatment modalities, recurrence rates, and permanent hypothyroidism were similar in both groups. Conclusion: With the COVID-19 pandemic, although there was a change in seasonal variation of SAT and an increase in the number of male patients, there was no change in the incidence and clinical course of SAT.

Background: Subacute thyroiditis (SAT) is an acute inflammatory disease of the thyroid. Mean platelet volume (MPV), neutrophil-to-lymphocyte ratio (NLR), and platelet-to-lymphocyte ratio (PLR) values determined from peripheral blood, are accepted as available and practical indicators of systemic inflammation. The purpose of this study was to evaluate the value of hematological parameters in the diagnosis and prognosis of subacute thyroiditis patients. Methods: This retrospective study included 306 SAT patients and 102 healthy control subjects. Retrospective analyses were made of age, gender, complete blood counts, C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), thyroid function tests, NLR, PLR, and thyroid volume of the patients and the results were compared with the control group. Results: The mean follow-up time of patients was 29.5 ± 14 months. The ESR, CRP, white blood cell (WBC) count, neutrophil count, hemoglobin, platelet counts, NLR, and PLR were significantly higher in the SAT patients. MPV and lymphocyte count were significantly lower in the SAT patients. The rates of recurrence and permanent hypothyroidism were 15.4% and 9.8%, respectively. The values of ESR, CRP, NLR, PLR, and MPV at the time of diagnosis were not determined to have any effect on recurrence or the development of permanent hypothyroidism. Conclusions: The results of this study showed higher PLR and NLR values in SAT patients compared with healthy control subjects, and a lower MPV value. These findings demonstrate that the assessment of hematological parameters in conjunction with radiological and clinical findings will assist in establishing an accurate diagnosis, especially in complicated SAT cases.