RESUMO
Riedels thyroiditis is a rare variant of thyroiditis that is characterized by replacement of the normal thyroid parenchyma by extensive fibrosis. Typically, the thyroid is diffusely involved and a painless, hard anterior neck mass shows clinical features similar to those of anaplastic thyroid carcinoma, that is, a rapidly enlarging, hard, fixed thyroid mass and symptoms such as dysphagia, dysphonia, and dyspnea. We encountered a case of Riedels thyroiditis in a 41-year-old female patient with a longstanding benign thyroid nodule for 6 years; she subsequently presented with a rapidly growing, hard, fixed, thyroid mass mimicking anaplastic thyroid cancer. The clinical features were indistinguishable from those of anaplastic transformation, but open biopsy excluded anaplastic thyroid cancer. After surgery, the final diagnosis of Riedels thyroiditis was made based on typical microscopic findings and immunohistochemical studies. We have reported this case and reviewed the related literature.