Sclerosing Forms of Autoimmune Thyroiditis: Hashimoto’s, Riedel’s, and IgG4-Related Forms

Abstract

Hashimoto’s thyroiditis (HT) has been considered for many years a well-defined clinicopathological condition. However, the recent observation, in some HT cases, of marked fibrosis and thyroid inflammation rich in immunoglobulin G4 (IgG4)-positive plasma cells, has led to the notion that subtypes of HT may exist. The increased IgG4-positive plasma cell infiltration suggests that this type of HT may have a close relationship with IgG4-related disease (IgG4-RD). This subgroup of HT, now called IgG4-thyroiditis, can be differentiated from non-IgG4 thyroiditis on the basis of clinical, sonographical, and serological findings. In addition, not only some cases of classical HT but also the well-known fibrous variant of HT has been included in the IgG4-thyroiditis spectrum. It is still under investigation whether Riedel’s thyroiditis (RT) as well may be considered as an IgG4-thyroiditis. As a matter of fact, RT is characterized by thyroid inflammatory lesions which involve the surrounding tissues, a finding not reported in IgG4-thyroiditis. However, according to some reports, RT with increased IgG4-positive plasma cells may represent the first clinical manifestation of an underlying IgG4-RD.