Riedel's thyroiditis is a rare chronic inflammatory disease of the thyroid gland first recognized by Bernhard Riedel in 1893. In order to clarify the clinical and pathologic features of this rare disorder, a recent case of Riedel's thyroiditis is presented, 6 additional cases from the surgical pathology files of the Cleveland Clinic Foundation are reported, and the 178 cases previously reported in the English literature are reviewed. The mean age at presentation has been 47.8 years, and 83% of patients have been female. Sixty-four percent were euthyroid, 32% hypothyroid, and 4% hyperthyroid. Antithyroid antibodies have been detected in 16 of the 25 patients tested. Since 1960, 34% of the published cases of Riedel's thyroiditis have reported the development of multifocal fibrosis. Immunohistologic study of the inflammatory infiltrate reveals a mixed population of B- and T-cells. Comparison with the infiltrate in Hashimoto's thyroiditis shows similar proportions of CD4- and CD8-positive lymphocytes. The relationship and origin of multifocal fibrosing lesions has not been defined, but an immunologic origin seems most likely. The finding of antithyroid antibodies in 67% of patients supports an autoimmune mechanism of injury.
