Subacute thyroiditis is an infection of the thyroid gland that occurs in the course of or immediately after contracting a viral infection, often affecting the first airways. This inflammation of the thyroid was classically associated with several viruses such as Coxsackie and Echoviruses, but never previously with Coronaviruses.
Subacute thyroiditis is associated with pain in the anterior region of the neck, where the thyroid gland is located, with extension even up to the ears. Patients may also complain of fever, trouble swallowing, muscle pain in addition to the classic symptoms of acute-onset hyperthyroidism(tachycardia, difficulty sleeping, nervousness, more rarely weight loss).
The symptoms of subacute thyroiditis are also associated with p>thyroiditis.
Laboratory examinations show increased indices of inflammation, a more or less important picture of hyperthyroidism, usually negativity of autoantibodies directed against the thyroid gland, while ultrasonography shows a fairly typical picture of inhomogeneity.
The natural history of subacute thyroiditis involves a phase of thyrotoxicosis (an increase in thyroid hormones) due to the inflammatory process that causes a release of preformed hormone into the circulation, followed by a phase of hypothyroidism that in 90% of cases resolves, leaving the patient in a condition of normal thyroid function.
This condition can develop in both patients with a severe form of COVID 19 and those with the mildly symptomatic (or completely asymptomatic) disease. In both cases, it appears to be underdiagnosed.
Susually, a subacute thyroiditis takes two to eight weeks to manifest, while in cases of post-COVID thyroiditis the development time seems to be three to five weeks.
The true incidence of subacute thyroiditis among those infected with SARS-CoV-2 is currently unknown. Some authors have suggested that up to 20% of patients with severe COVID 19 infection may develop thyroid problems.
In contrast, other pre-pandemic and pandemic studies, which analyzed the incidence of subacute thyroiditis in areas with an increased incidence of COVID-19 infection, did not report a higher rate of disease.
In a study published in The Lancet in 2020 and carried out at the Policlinico di Milano, the prevalence of subacute thyroiditis in patients admitted to intensive care units (ICUs) was found to be 0.5% of inpatients in 2019 (a percentage that tracks that of the general population), while the prevalence of subacute thyroiditis in ICUs in 2020 was around 10% of inpatients.
Diverse studies have also noted a possible association between COVID vaccines and subacute thyroiditis. It would develop between 7 and 90 days after COVID vaccine while in other cases, such as following influenza vaccine (H1N1) or hepatitis B vaccine, it usually takes between 2 days and 8 weeks to occur. Inflammatory indices, ultrasound findings, and disease-related hypothyroidism data are very similar between patients with classic subacute thyroiditis and those who developed it following COVID vaccination.
Once recognized, subacute thyroiditis requires no special approach when compared with cases not associated with COVID-19. Complete resolution of thyroid hormone abnormalities and inflammation is expected.
Only a small percentage of individuals develop permanent hypothyroidism; however, the rate seems similar to that of the pathology associated with other viral infections (non-COVID 19).
Why does this association exist?
.The SARS-CoV-2 virus is able to enter cells rich in the ACE2 (angiotensin-converting enzyme) receptor, and thyroid cells also belong to the latter; therefore, thyroid cells are a potential target for SARS-CoV-2 entry.
In addition, another mechanism underlying the association between COVID-19 and subacute thyroiditis could be the so-called cytokine storm (especially interleukin-6, IL-6) that develops during SARS-CoV-2 infection.
Considering the importance of thyroid function, particularly in already cardiovascularly compromised individuals, evaluation of thyroid function in hospitalized individuals is recommended.
Sources:
I.Muller, The Lancet, diabetes and endocrinology, 2020, 8 (9): 739
