Subacute thyroiditis - Is it really linked to viral infection? Retrospective hospital patient registry study.

Objective: Subacute thyroiditis (SAT) is a painful inflammatory disorder of the thyroid gland, which - after a phase of thyrotoxicosis - leads to transient, or less frequently permanent hypothyroidism. Apart from a strong association with specific HLA alleles, the causes are uncertain. Viral disease has been hypothesised as a trigger, with Enteroviruses, namely Echoviruses and Coxsackieviruses, showing a seasonal distribution that coincides with the incidence of SAT. In the first year of the COVID-19 pandemic, strict hygiene measures led to a sharp decline in infections and thus offered the opportunity to test this hypothesis.

Methods: We analysed national registry data of hospitalised patients from Germany during the years 2015 to 2022 (Federal statistical Office (Destatis), Wiesbaden, Germany) and surveillance data on infectious diseases from the same years (clinical-virology.net and RKI). Statistical analysis includes modelling of seasonality by month, polynomial autoregression and Granger causality to assess dependency of future SAT frequencies from past ones, and association of virus incidence to SAT frequency, respectively.

Results: Our study confirms previously described epidemiological findings with higher incidence in females and a seasonal peak in late summer coinciding with the seasonality of enteroviruses until 2019. In 2020, the pattern remained unchanged, except for the marked reduction of enteroviruses and other pathogens (except SARS-CoV-2) due to hygienic measures. Moreover, the SAT seasonality in the years 2021 and 2022 was apparently unaltered through the COVID-19 pandemic.

Conclusions: Our study provides strong evidence that despite their seasonal pattern, Echoviruses and Coxsackieviruses are not the cause of SAT. Moreover, no other analysed virus (including Influenza A and B, Parainfluenza, Rhinovirus, Human Coronaviruses including SARS-CoV-2) showed any association.