Thyroiditis refers to inflammation of the thyroid gland and is not a single disease, but a large group of diseases with various clinical classifications and different etiologies and prognoses. It is divided into acute septic thyroiditis, subacute thyroiditis and chronic thyroiditis according to the speed of onset. Subacute thyroiditis is further divided into subacute granulomatous thyroiditis, also known as subacute thyroiditis, and subacute lymphocytic thyroiditis (painless thyroiditis), which is further divided into sporadic thyroiditis and postpartum thyroiditis. In contrast, chronic thyroiditis includes chronic lymphocytic thyroiditis (Hashimoto’s thyroiditis) and chronic fibrous thyroiditis. By etiology, they can be classified as bacterial, viral, autoimmune, post-radiation, parasitic, tuberculous, syphilis and AIDS infections. The most common clinical thyroiditis are chronic lymphocytic thyroiditis and subacute granulomatous thyroiditis, and painless thyroiditis is often seen clinically; the most common from an etiologic perspective is autoimmune thyroiditis. Most thyroiditis symptoms are similar, except for painful enlargement due to inflammation of the thyroid gland itself, which usually undergoes a process of transformation from hyperthyroidism to hypothyroidism. Because inflammation destroys the thyroid follicles, stored thyroid hormones leak into the bloodstream and symptoms of hyperthyroidism appear. However, because the destroyed follicles cannot continue to secrete, they can again become hypothyroid due to a lack of thyroid hormones. In some types of thyroiditis follicular function can be restored, but in others it becomes lifelong hypothyroidism. Therefore, thyroiditis is a large group of diseases with multiple causes, each type having its own etiology and pathogenesis, and treatment and prognosis varying.