Subacute thyroiditis refers to a temporal classification of the different forms of thyroiditis based on onset of symptoms. The temporal classification of thyroiditis includes presentation of symptoms in an acute, subacute, or chronic manner. There are also other classification systems for thyroiditis based on factors such as clinical symptoms and underlying etiology.

Broadly, there are three categories of thyroiditis that can present in a subacute fashion, including subacute granulomatous thyroiditis, subacute lymphocytic thyroiditis, and drug-induced thyroiditis. In all three categories, there is inflammation of the thyroid gland causing damage to the thyroid follicular cells which produce and secrete thyroid hormone. This often results in three phases of thyroid dysfunction beginning with initial thyrotoxicosis followed by hypothyroidism before resolution back to normal thyroid function. In the thyrotoxic stage, individuals usually complain of fever, myalgia, and may have associated anterior neck pain among other symptoms. In the hypothyroid stage, they may be asymptomatic or experience mild symptoms. In most cases, the thyroid dysfunction is transient and people recover with symptomatic treatment.

Among the different classification systems for thyroiditis, the onset of symptoms can be used to classify thyroiditis as acute, subacute, or chronic. The three types of thyroiditis that can occur in a subacute manner include subacute granulomatous thyroiditis, subacute lymphocytic thyroiditis, and drug-induced thyroiditis.

Subacute granulomatous thyroiditis affects more women than men and often follows a viral upper respiratory tract infection with many implicated viruses including Coxsackievirus, Epstein-Barr virus, and recently SARS-CoV-2 among others.

Subacute lymphocytic thyroiditis, also called painless or silent thyroiditis, occurs in individuals with underlying autoimmune disease or after pregnancy. It is considered to be a variant of Hashimoto's thyroiditis. When subacute lymphocytic thyroiditis occurs up to 12 months postpartum, it is called postpartum thyroiditis. It has an increased incidence in women with presence of thyroid peroxidase (TPO) antibodies prior to pregnancy and in women with preexisting Type 1 diabetes. Postpartum thyroiditis can recur in subsequent pregnancies.

Drug-induced thyroiditis can occur in individuals receiving certain therapies such as amiodarone, interferon-alpha, lithium, tyrosine-kinase inhibitors, and immunotherapies among other drugs. These individuals may develop subacute lymphocytic thyroiditis.

In all three types of thyroiditis, there is inflammation of the thyroid gland which leads to clinical symptoms in three phases. The initial thyrotoxic phase is followed by hypothyroid phase before reaching a euthyroid phase where normal thyroid function in restored. The timing of each phase varies and not every person experiences all three phases. In the thyrotoxic phase, the follicular cells of the thyroid gland are damaged leading to release of pre-formed thyroid hormone and symptoms of thyrotoxicosis. The damage to the follicles impairs ability to produce and secrete thyroid hormones which then leads to a hypothyroid phase. As inflammation resolves, the thyroid follicles regenerate and begin to produce thyroid hormones once again.

In subacute granulomatous thyroiditis, there is infiltration of neutrophils and other immune cells which cause damage to the thyroid follicular cells. The damage leads to formation of granulomas with multi-nucleated giant cells and associated fibrosis. In contrast, subacute lymphocytic thyroiditis is characterized by an infiltration of lymphocytes causing damage to the thyroid gland, similar to Hashimoto's thyroiditis.