Objectives: The objective of this study was to collect regional data of Upper Franconia and observe the association between thyroid parameters thyroid-stimulating-hormone (TSH), free triiodothyronine (fT3), free thyroxine (fT4) and an infection with SARS- CoV- 2. It was set to investigate the prevalence of patients with abnormal thyroid parameters (outside the range of reference), the association of COVID-19 infection on thyroid gland secretion based on laboratory parameters of routine clinical data and a possible increased vulnerability of persons with a history of Hashimoto's thyroiditis and Graves' disease to infection with COVID-19. Patients and methods: For this retrospective observational study, routine clinical data of patients admitted to the REGIOMED Hospital Coburg, Upper Franconia region in Bavaria, Germany (single-site) were included and subsequently analyzed. The admissions were in between the official pandemic declaration of WHO on 11th March and ended with the declaration of German government on 07th April 2023 ending the pandemic. Data was collected from anonymized sources. A total of 10 450 cases were included of which 195 were also diagnosed with Hashimoto´s thyroiditis. Furthermore, 93 were included based on their ICD code in the group for Grave´s disease. The statistical analysis was carried out with RStudio. Results: The first and foremost finding of this study indicates that the prevalence of a TSH value below the reference was significantly lower in the group with an active COVID infection in comparison to the non-COVID group. Other laboratory parameters fT3 and fT4 did not change in a mild course of an infection. Secondly, reduced deiodinase activity was seen to be caused by the CoV-19 infection and not by autoimmune activity in the setting of Hashimoto´s thyroiditis. Furthermore, neither Hashimoto´s thyroiditis nor Graves´ disease were seen to be associated with severely altered thyroid levels during a CoV-19 infection. Conclusion: A mild infection with COVID-19 increases the risk of a lower TSH value but no altered thyroid function was seen in these patients. Following the pandemic several studies in the realm of the thyroid gland have been conducted but found widely heterogenic and also conflicting results have been found. Furthermore, a second significant finding of this study described reduced levels of SPINA GD with a subsequently a lowered deiodinase activity. This finding was indicated to be caused by the CoV-19 infection and not by an autoimmune activity in the setting of Hashimoto´s thyroiditis. Targeted prospective studies need to follow to evaluate on associations found in our study.