It is well known that an infection can be a triggering factor for the development of autoimmune thyroid disease. The COVID-19 pandemic was an example of this, with several reports of both subacute thyroiditis as well as Graves' disease occurring after experiencing SARS-CoV-2 disease. There have also been cases of Graves' disease reported after COVID-19 vaccination administration (<4 weeks between the vaccine and the development of hyperthyroidism). However, the current evidence is of low quality and consists mainly of case reports. A search strategy on October 3, 2022, in the medical database PubMed could only retain one retrospective study on this subject. In this study by di Filippo et al., approximately 64 new cases of Graves' disease with hyperthyroidism were identified in the Milan region, Italy, in the year 2021, of which 20 patients had an onset within 4 weeks following COVID vaccine administration (31.2%).

The investigators would like to increase the knowledge about the possible link between COVID-19 (both the disease and the vaccination) and Graves' disease, by means of a case-control analysis of all 'de novo' cases of Graves' hyperthyroidism described in the C.H.U. Brugmann. The investigators want to investigate whether Graves' disease after vaccination would be clinically different from the "classic" Graves' disease, thereby describing factors such as the duration of the disease, the level of thyroid-stimulating immunoglobulins (TSI), the percentage of T3-dominant Graves' disease, or the dose of thyreostatics (such as strumazole) required to control the disease.

Inclusion Criteria:

All patients who developed 'de novo' Graves' disease between March 2020 (the beginning of the COVID-19 epidemic in Belgium) and October 2022.

Exclusion Criteria:

• Pregnancy
• Pre-existing Graves' disease (before March 2020)
• Diagnosis of other thyroid disease, such as subacute or painless destructive thyroiditis
• Use of thyroid-interfering drugs, such as lithium, amiodarone or immune checkpoint inhibitor therapy

All patients who developed 'de novo' Graves' disease between March 2020 (the beginning of the COVID-19 epidemic in Belgium) and October 2022. These patient cases will be identified based on the biochemical-radiological factors diagnostic for "classic" Graves' disease, i.e., hyperthyroidism in the context of positivity for TSI and/or homogeneous captation on thyroid scintigraphy