Simultaneous involvement of thyroid by Riedel's [correction of Reidel's] disease and fibrosing Hashimoto's thyroiditis: a case report.

We report an unusual thyroid lesion showing histologic features of both Riedel's [corrected] disease and fibrosing Hashimoto's thyroiditis in a 57-year-old white female. The clinical presentation was hypothyroidism associated with a solitary firm to hard cold nodule replacing the entire right lobe of thyroid gland. Pathological examination demonstrated extensive replacement of the thyroid parenchyma with dense keloidal fibrosis, intermixed well-developed lymphoid follicles, scattered lymphocytes, and plasma cells. The fibrotic process extended into the perithyroidal soft tissues and skeletal muscle with complete obliteration of the thyroid capsule. These findings were consistent with Riedel's [corrected] disease. However, the immunohistochemical stains for B and T markers and immunoglobulin light chains showed an immunoprofile consistent with Hashimoto's thyroiditis. This combination of Riedel's [corrected] disease and fibrosing Hashimoto's thyroiditis is rare and coincidental, as both represent two distinct clinicopathological entities.