Although
IgG4-related disease is characterized by extensive infiltration of IgG4-positive plasma cells and lymphocytes of various organs, the details of this systemic disease are still unclear. We screened serum total
IgG levels in the patients with
Hashimoto thyroiditis (HT) to illustrate the prevalence of IgG4-related
thyroiditis in HT. Twenty-four of 94 patients with HT (25.5%) had elevated serum
IgG levels and their serum
IgG4 was measured. Five of the 24 cases had more than 135 mg/dL of
IgG4, which is the serum criterion of
IgG4-related disease. One was a female patient who was initially treated as
Graves' disease and rapidly developed a firm
goiter and
hypothyroidism. The biopsy of her thyroid gland revealed that follicular cells were atrophic with squamous
metaplasia, replaced with
fibrosis, which was compatible with the fibrous variant of HT. Immunohistochemical examination revealed diffuse infiltration of IgG4-positive plasma cells, and the serum
IgG4 level was 179 mg/dL. The levels of
IgG and
IgG4 were positively correlated with the titers of
anti-thyroglobulin antibody or anti-
thyroid peroxidase antibody. In conclusion, at least a small portion of patients with HT with high titers of anti-thyroid
antibodies may overlap the IgG4-related
thyroiditis.