This article describe the case of a 38-year-old woman with simultaneous involvement of the thyroid by Hashimoto's thyroiditis and Riedels's disease, associated with retroperitoneal fibrosis and lipidic endarteritis. According to a large review of the literature on the occurrence of this rare condition the difficulty in sharply defining the two thyroid processes is discussed. The chronology of the events is analyzed, and etiopathogenic hypotheses are listed with emphasis on the relationship between the vascular lesions and the onset of the fibrosing process.