RESUMEN
Riedel's thyroiditis appears in the form of a hard cervical mass with rapid onset, and it is associated with extensive fibrosis that compresses nearby structures, such as the trachea and supra-aortic vessels; its diagnosis is essentially histopathological. Although its histological characteristics have been well established, there are some diagnostic pitfalls. We report here the case of a 37-year-old woman, with clinical and histopathological data suggesting Riedel's disease. Fibrosis regressed after treatment with corticosteroids, relieving the compressed airways. However, in contrast with the latest knowledge on this disease, the IgG4 serum levels were consistently normal, and positron emission tomography in search of extensive fibrosis revealed an abnormal metabolic activity of the bone marrow. The final diagnosis revised by the histopathologist was that of nodular sclerosing Hodgkin's lymphoma. This case allows us to review the diagnostic approach when facing a thyroid mass with extremely rapid evolution.