Introduction

Subacute thyroiditis (also called as De Quervain’s thyroiditis, viral thyroiditis, subacute granulomatous thyroiditis, or giant cell thyroiditis) is a self-limiting inflammatory disorder of the thyroid gland which usually follows or coexists with a viral infection [1]. The etiology of SAT has been linked to viral infections such as mumps, measles, rubella, coxsackie, and adenovirus, either through direct viral toxicity or inflammatory response against the virus [2, 3]. It generally presents in women with neck or jaw pain, tender thyroid gland, and systemic signs/symptoms [2]. As of February 3, 2021, WHO on its official website has reported more than 103 million documented cases of COVID-19 while the death toll is above 2.2 million. Efforts are being made to identify the endocrinological effects of COVID-19, and abnormalities of hypothalamic-pituitary-thyroid (HPT) axis have been reported [4]. The interplay between thyroid hormones and immune system along with the direct cytotoxic effect of the virus is proposed to play a role in these abnormalities, including subacute thyroiditis [4]. The increasing evidence of SAT during or after the COVID-19 infection has been reported in various published cases [5,6,7,8,9,10,11,12,13,14,15,16,17], which highlights the concern that physicians ought to consider COVID-19 (ongoing or previous) as an etiological factor/trigger in patients presenting with subacute thyroiditis. This is especially important amidst the ongoing pandemic or even in the future as physicians encounter SARS-CoV-2. All these factors, coupled with lack of a published systematic review on this topic, encouraged us to undertake this write-up.

Methods

Search Strategy

Online databases including PubMed/Medline and Google Scholar were searched for articles published until February 3, 2021. Search strategy followed Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines by using keywords like the following: COVID-19, SARS-CoV-2, Subacute Thyroiditis, De Quervain’s Thyroiditis, and Viral Thyroiditis. All studies, regardless of time, language, and country of publication, and all types of research articles were scrutinized. The bibliographies of individual case reports and case series were also scrutinized to find any relevant cases. The final available references were downloaded into an EndNote library. The complete strategy is outlined in the PRISMA flowchart (Fig. 1).

Fig. 1
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PRISMA flow diagram

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Study Selection

The relevant published articles on this topic were mainly case reports and case series. Joanna Briggs Institute Critical Appraisal Tool [18] was employed by two authors independently to assess the quality of all case reports and case series. Both the authors engaged in discourse to reach one consensus score for each article. One case report was translated from Portuguese into English via Google Translator, but it was excluded due to a poor score on Critical Appraisal. All the other case reports and case series included in the final analysis were in English language. Data were curated and organized in the form of two tables each for case reports and case series. One table focused on patient demographics, clinical features, treatment(s), and follow-up/outcome. The other table focused on lab investigations and imaging results. Continuous variables were presented as means ± standard deviations, and categorical variables were presented as absolute values and percentages. Microsoft Excel was used for data extraction and statistical analysis for this study.

Results

Our search identified 30 articles; 9 were excluded due to duplication, and 7 articles on COVID-19 were excluded because they did not address subacute thyroiditis and 1 was excluded because it scored low on Critical Appraisal. Finally, 13 articles—11 case reports and 2 case series [5,6,7,8,9,10,11,12,13,14,15,16,17]—were included in the final analysis. Since 10 patients were described in the two case series, individual data from 21 total patients is described here, in the form of 2 tables each for case reports (Tables 1 and 3) and case series (Tables 2 and 4). The mean age of patients was 40.0 ± 11.3 years (range 18–69 years). Out of the 21 patients, 15 were females (71.4%) and 6 were males (28.6%). Most cases were reported in Italy (n = 7; 33.3%) and Iran (n = 6; 28.6%), 2 in the USA (9.5%) and Turkey (9.5%) each. One case (4.8%) each was reported in Singapore, Mexico, the Philippines, and India. Out of the 21, 9 reported a contact/travel history (42.9%), 4 reported no contact history, whereas the remaining studies did not mention whether or not exposure had occurred. Only 2 reports mentioned a comorbidity, with one patient having long-standing diabetes, and 1 having asthma. One patient had a family history of thyroid disease, whereas 2 patients had a goiter long before the onset of any symptoms.

Table 1 Demographics, clinical presentation, course, and outcome of COVID-19-associated subacute thyroiditis (review of case reports)
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Table 2 Demographics, clinical presentation, course, and outcome of COVID-19-associated subacute thyroiditis (review of case series)
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Table 3 Diagnostics and laboratory investigations of COVID-19-associated subacute thyroiditis (review of case reports)
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Table 4 Diagnostics and laboratory investigations of COVID-19-associated subacute thyroiditis (review of case series)
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Mean days between the start of COVID-19 illness and the appearance of SAT symptoms were 25.2 ± 10.1 (minimum 5 days and maximum 42 days). Five patients had ongoing COVID-19, whereas the infection had resolved in 16 patients before onset of SAT symptoms. COVID-19 diagnosis was made by RT-PCR swabs in 12 patients (57.1%), whereas in 9 cases (42.9%) with no current symptoms, no previous RT-PCR, or a negative current RT-PCR, a past COVID-19 illness was confirmed by the presence of IgG/IgM against SAR-COV2 in the patients’ blood. Out of the 13 people who had a chest X-ray/CT chest at presentation for SAT, only 3 showed abnormalities (21.3%).

Fever and neck pain were the most common presenting complaints with each being present in 17 out of 21 patients (81%). Other complaints included fatigue, palpitations, odynophagia, sweating, and throat pain. Out of the 16 reports that mentioned neck examination, a tender thyroid was found in all 16 patients (100%). Out of the 21 patients, 17 mentioned (94.4%) the presence of hyperthyroid symptoms like fatigue and palpitations, 1 mentioned absence of such symptoms, and 3 reports did not mention anything in this regard. Out of the 21, 15 cases reported ESR (erythrocyte sedimentation rate) values which were high in all 15 patients (100%), with a mean of 78.5 ± 28.8 mm/h. Eighteen cases reported CRP (C-reactive protein) values, and it was elevated in all 18 (100%), with an average 39.0 ± 30.3 mg/L. Deranged TFTs (thyroid function tests) were seen in all patients, with each case having low TSH, and either high T3 or high T4 or both. Serum thyroglobulin was detectable in only 3 patients. Antithyroid antibodies were only reported positive in 2 patients, one being TPO-Ab (thyroid peroxidase antibody) and the other one being TgAb (thyroglobulin antibodies). All 21 cases (100%) had abnormal thyroid ultrasounds suggestive of SAT.

Twenty case reports discussed the medications of their patients; 18 out of these were administered steroids while others were given hydroxychloroquine, ibuprofen, methimazole, or only oral aspirin. Four of these were also given beta-blockers for controlling hyperthyroid symptoms. All patients recovered a few days after treatment; however, 5 patients were reported to have developed hypothyroid features/TFTs on follow-up after resolution of SAT symptoms.

Discussion

To our knowledge, this is the first systematic review of subacute thyroiditis (SAT) in COVID-19 patients. The term thyroiditis literally means thyroid inflammation. Most thyroidologists classify thyroiditis into (a) infectious thyroiditis (includes all forms of infection, except viral); (b) subacute thyroiditis; (c) autoimmune thyroiditis (Hashimoto’s thyroiditis and Grave’s disease); and (d) Riedel’s thyroiditis [3]. Among all types of thyroiditis, subacute thyroiditis has been most strongly linked to viral infections in the literature [3]. The viral particles presumed to be of influenza or mumps were first demonstrated in the follicular epithelium of a patient suffering from subacute thyroiditis [19], and multiple viruses have been implicated as a trigger for subacute thyroiditis since then. Some of them include mumps, measles, rubella, influenza, coxsackie, adenovirus, varicella zoster virus, cytomegalovirus, Epstein-Barr virus, hepatitis E, and HIV [2, 3, 20]. In most patients, a typical viral prodrome of malaise, myalgias and fatigue is also seen clinically. [21] A variety of case reports and case series have emerged over the last year suggesting that COVID-19 virus may also act as a possible trigger for SAT, either during the infection or after its resolution. While large-scale clinical data needs to be reviewed before making strong comments, the recurring and consistent evidence from the recognition of this virus to date suggests that COVID-19 may be another virus that merits the list of viral culprits in subacute thyroiditis.

SAT typically presents as pain localized to the anterior of the neck that may radiate up to the jaw or ear on either side, low-grade fever, fatigue, and mild thyrotoxic/hyperthyroid symptoms [2, 22,23,24,25,26]. Tenderness on palpation and enlarged size of the gland are some of the cardinal signs. Laboratory findings suggest hyperthyroid etiology (suppressed TSH, low free T4, and poor or no thyroid uptake), and thyroid ultrasound shows characteristic findings of poorly defined hypoechoic areas with a heterogeneous echo pattern [27,28,29]. Inflammatory markers like ESR and CRP are typically elevated. The disease may have a triphasic clinical course of hyperthyroidism, hypothyroidism, and return to normal thyroid function. The hypothyroid phase, however, may last for months until the patient becomes euthyroid [24,25,26]. Most of the cases we reviewed presented with findings consistent with the above mentioned clinical features and investigations. Majority of the patients presented with complaints of anterior neck pain and fever, while some patients also presented with generalized fatigue, sore throat, cough, and odynophagia. In a total sample of 21, 16 patients (76%) gave a positive history of a previous COVID-19 infection confirmed either by a previous RT-PCR or a positive antibody test (IgM/IgG). The remaining 24% developed SAT during ongoing COVID-19 infection; however, two patients [5, 12] had an asymptomatic infection which was only confirmed via RT-PCR after they presented with SAT features.

It is believed that SARS-CoV-2 can affect the thyroid function in multiple ways. The three reported effects of the virus are (a) thyrotoxicosis (either subacute/painful thyroiditis or painless/atypical thyroiditis); (b) hypothyroidism (central or primary); and (c) nonthyroidal illness syndrome (previously known as euthyroid sick syndrome) [4]. This suggests that the effects of the virus on thyroid gland are highly variable and it is difficult to predict the abnormalities in thyroid function tests (TFTs). However, when studying the complication of subacute thyroiditis in COVID-19 alone, a strikingly similar pattern in terms of presentation, TFTs and outcome is observed.

Cytokine storm syndrome—a term used to describe the detrimental effects of hypercytokinemia on human cells—has been well described in the literature as the cause of thyroid problems [30]. While it is well documented that this storm is the cause of nonthyroidal illness syndrome seen in COVID-19 patients, at present there is little evidence to suggest the direct thyroid cytotoxic effect of cytokines, at least in humans [30, 31]. Immune-mediated post-viral inflammatory reaction, involving both the adaptive and innate immune systems, has also been described in the literature as a cause of thyroid problems [32, 33]. This mechanism might be responsible for the post-infection SAT observed in the majority of patients described here too.

Among the many possible mechanisms, direct viral damage remains the most reliable evidence to date. The molecular interaction of SARS-CoV-2 with ACE-2 and TMPRSS2 receptor is required for entry into the human cells [34,35,36] and recent work by Rotondi et al. and Lazartigues et al. demonstrated the presence of ACE-2 and TMPRSS2 mRNA in thyroid cells [37, 38]. This receptor-virus interaction is similar to previously described viruses of the same family, SARS-CoV and MERS-CoV [39, 40]. The abundance of these receptors in thyroid [41] compared to other tissues (small intestine, heart, adipose) potentially explains the subacute thyroiditis associated with SARS-CoV-2, hence strengthening the documented mechanism of direct viral injury in SAT. Moreover, the structural proximity of the virus-laden superior airway to the thyroid gland may play some role as well [42]. This mechanism—direct follicular cell damage leading to spillage of thyroid hormones into plasma—probably explains the predominant thyrotoxic clinical picture (palpitations, tachycardia, insomnia, anxiety, etc.) in more than ¾ and thyroid function tests suggesting hyperthyroid etiology in 100% of the cases. It has also been hypothesized that drugs used in COVID-19, especially glucocorticoids and low molecular weight heparin, may also damage the gland and affect thyroid function [43]. While this mechanism might play a role in thyroid abnormalities, there is not enough evidence to support it in our review because not all patients described here received steroids and heparin for COVID-19 infection.

In the cases we reviewed, more than ¾ patients (76%) patients were females as is seen in most cases of subacute thyroiditis. The female preponderance also highlights the autoimmune etiology of SAT that is well documented in the literature [2, 44, 45]. It is important to realize that patients can either present with pre-dominant COVID-19 features (headache, anosmia, upper respiratory symptoms) or pre-dominant SAT features (given above). After a careful review of the literature, the authors reiterate that anterior neck pain must carefully be assessed through a good history and examination so that it is not conflated with upper respiratory symptoms.

Keeping in view the above discussion, we believe it is important for clinicians to assess for SAT when encountering patients with ongoing or previous COVID-19. It is also imperative to assess for an underlying asymptomatic infection via RT-PCR if a patient presents with features suggestive of SAT. A proper follow-up of COVID-19 might also be needed because most cases have been reported in those with a recent or previous infection. It must also be noted that treatment outcomes in all the 21 cases were excellent with steroids and anti-inflammatory drugs, reiterating the need for timely diagnosis of this clinical entity.

The authors would like to acknowledge some limitations while drawing inferences in this review. Firstly, the sample size is small because of lack of published literature on the topic. Secondly, the lack of control group and individual-centered data limits the generalizability of results. Lastly, there is a tendency towards publication bias as clinically challenging cases are more likely to be reported and published.

Conclusion

Direct viral injury and post-viral inflammatory reaction may contribute to subacute thyroiditis seen during or after COVID-19 infection. The mechanism of direct viral injury is well supported by the presence of ACE-2 and TMPRSS2 mRNA in thyroid cells, while post-viral inflammatory reaction has been documented previously as the cause in many other viral infections associated with subacute thyroiditis. COVID-19-associated SAT is likely to present with the classic clinical features of fever and neck pain, hyperthyroid TFTs (low TSH, high free T4), and suggestive ultrasound findings. The recognition of this clinical entity is important for physicians to consider because prompt treatment is likely to lead to complete resolution; however, the possibility of a hypothyroid phase after SAT treatment should not be ignored. A proper follow-up after COVID-19 resolution is necessary because cases of SAT have been reported months after the infection.