Warthin-like papillary thyroid carcinoma with immunoglobulin G4-positive plasma cells possibly related to Hashimoto's thyroiditis.

Hashimoto's thyroiditis with heavy lymphoplasmacytic infiltration is a common comorbidity of immunoglobulin G4 (IgG4)-related thyroiditis and Warthin-like papillary thyroid carcinoma (WL-PTC). We hypothesized that WL-PTC may have a strong association with IgG4-related thyroiditis. To validate this hypothesis, we clinically and immunohistochemically studied 17 WL-PTC cases. Fourteen patients (82.4%) had anti-thyroglobulin antibody and were confirmed to have Hashimoto's thyroiditis through microscopic analysis. Among them, five (29.4%) had disease consistent with IgG4-related thyroiditis but did not exhibit a "storiform" pattern or obliterative phlebitis. IgG4-related diseases were not found in other organs. No cases with serum IgG4 level of >135 mg/dL were noted. A total of 94.1% of WL-PTC cases had IgG4-positive plasma cells (PCs) in the stroma, and cases with rich IgG4PCs were more frequently associated with Hashimoto's thyroiditis than those with poor IgG4PCs. In this study, all three cases without Hashimoto's thyroiditis had poor IgG4PCs, and one of them did not exhibit IgG4PCs in the stroma of WL-PTC and Hashimoto's thyroiditis. Nodal metastatic lesions were seen in eight cases, all of which were not WL-PTC. As such, we should consider that the Hashimoto's disease with rich IgG4PCs seen in our cases is representative of non-IgG4-related disease and not IgG4-related disease involving multiple organs. This study is the first to demonstrate the presence of IgG4PCs in the stroma of WL-PTC. We concluded that the appearance of IgG4PCs in the stroma of WL-PTC may be related to Hashimoto's thyroiditis with rich IgG4PC.