Hashimoto's thyroiditis: old concepts and new insights

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Hashimoto's thyroiditis is commonly considered as a well defined clinicopathological entity. Its diagnosis and treatment have changed little over the last few decades. This review examines the recent progress in understanding Hashimoto's thyroiditis, particularly with regard to its close relationship to IgG4-related systemic disease (IgG4-RSD). During the 1-year review period, new studies have reported that there is a unique subtype of Hashimoto's thyroiditis, termed IgG4 thyroiditis, which is histopathologically characterized by lymphoplasmacytic infiltration, fibrosis, increased numbers of IgG4-positive plasma cells, and high serum IgG4 levels, which indicate that this group of Hashimoto's thyroiditis is closely related to IgG4-RSD. Furthermore, IgG4 thyroiditis and non-IgG4 thyroiditis present different clinical features, with IgG4 thyroiditis being more closely associated with rapid progress, subclinical hypothyroidism, higher levels of circulating antibodies, and more diffuse low echogenicity. In addition, Riedel's thyroiditis was recently demonstrated to be a thyroid manifestation in patients with systemic IgG4-RSD, which calls for the definition of IgG4 thyroiditis to be expanded. New insights into Hashimoto's thyroiditis with special reference to IgG4-positive plasma cells offer a novel way of viewing this well defined disease. IgG4-RSD occurring in the thyroid gland may involve two different manifestations: the organ-specific Hashimoto's thyroiditis type and the systemic Riedel's thyroiditis type, which share similar IgG4-related sclerosing features.