Immunoglobulin G4-related sclerosing disease (IgG4-RSD) represents a recently 
identified inflammatory disorder in which infiltration of IgG4 plasma cells 
causes fibrosis in organs. While IgG4-RSD is well documented in the pancreas and 
other organs, it is poorly characterized in the thyroid gland. We report a case 
of a 48-year-old female with a fibrotic thyroid mass associated with a 
retroperitoneal fibrosis. Diagnosed early as Riedel disease, the high serum 
IgG4, immunohistopathology and decreased fibrosis with corticosteroid therapy, 
finally confirm for the first time, the origin of IgG4-RSD fibrosis of the 
thyroid.