The results of the present study show a similar incidence of subacute thyroiditis over the last 5-years. In particular, throughout the 7-month period concomitant to the outbreak of Covid-19 infection, no enrichement in the diagnosis of SAT was registered. This observation appears strengthened by the fact that the present study was carried out in the Brescia areas, the region with the highest prevalence of Sars-CoV-2 infection in Italy [1]. It should be noted that, a previous diagnosis of Covid-19 was detected in one patient out of 10 developing SAT from April 2020 to October 2020.
De Quervain's thyroiditis shows a typical seasonal variation in its incidence with a higher prevalence in spring and summer [5]. Thus, the present study was specifically designed to take into account the number of SAT diagnosis rendered in the April-October period of different years.
According to the Rochester Epidemiology Project, SAT is estimated to display an incidence of 12.1 cases per 100,000/year, 5 times higher in women than in men [6], and it is presumed to be caused by a viral infection or a postviral inflammatory process possibly associated to entero, coxsackie or adenovirus [3, 7].
It is interesting noting, that these data are very closed to what observed in the Brescia areas through the last years. Indeed, on a population of about 200.000 inhabitants we should have 24 cases/years. Thus, the effective number of cases observed in seven months this year as well as in the previous ones are indeed those expected in Brescia areas yearly.
Brancatella and colleagues recently described the first case of SAT in a patient affected by SARS-CoV-2 [8]; the Authors alerted “clinicians to additional and unreported clinical manifestations associated with Covid-19” on the basis of a chronological association between Covid and SAT disease observed in that patent. Since then, several other case reports were published [9-16].
The pathophysiology of the association between SAT with Covid-19 infection is reported to be similar to the association with other viral conditions targeting the thyroid of genetically susceptible individuals. Muller et al. suggested that this could be due to the presence of the angiotensin-converting enzyme 2 (ACE2) receptors which are more prevalent in thyroid than lung cells [17]. ACE2 receptors are present in different tissue such as myocardium, gastrointestinal mucosa and respiratory tract [18, 19], all tissue shown to harbour Covid-19 virus [20, 21]. More recently, Rotondi et al, demonstrated that the mRNA encoding for the ACE2 receptor is expressed in thyroid follicular cells in human thyroid surgical specimens and in primary cultures of thyroid cells, making them a potential target for Sars-Cov-2 [22]. Taken together the above evidences, the hypothesys that Sars-Cov-2 could be responsible for subacute thyroiditis appears plausible. However, if Covid-19 would represent a major clinical condition for subsequent SAT development, due for example to a specific tropism of this virus for the thyroid, one would expect an increase of SAT incidence during the Covid-19 outbreak. At difference with this statement, we did not observe any increase in the incidence of SAT during the 7-month period of 2020 as compared to similar periods over the last years. Furthermore, no differences in the male/female ratio nor in the clinical phenotype at presentation were observed in patients developing SAT in “the Sars-Cov-2 year”.
Another and likely more intriguing aspect could be due to the possibility that Covid-19 patients have assumed corticosteroids masking a contemporary subacute thyroiditis. Unfortunately, we have no possibility to demonstrate or rebut this possibility. In addition, it could be theoretically possible that few patients with SAT were cured by their family physician and did not attend the endocrine out-patient clinic. However, the above possibility should be considered of negligible relevance in view of the fact that from April 2020, an endocrinological remote consultation for patients and physicians was established and active and no query or cases of SAT were received. Thus, although potential limitations might be present, the here reported results appear reliable.
In conclusion, the possibility that Covid-19 could be responsible for the development of subacute thyroiditis, although theoretically possible and documented by several case reports, appears not to be a frequent event. The here reported data do not demonstrate an increase of the incidence of SAT in Brescia areas, a region with the highest prevalence of Covid-19 in Italy during the period of the pandemy outbreak.