- Riedel thyroiditis, a rare disorder associated with IgG4 characterized by extensive fibrosis involving thyroid & contiguous neck structures. - The presence of a hard & fixed thyroid mass clinically simulates a thyroid carcinoma; but unlike thyroid carcinoma it is seen in young pts. - It may be associated with fibrosis in other sites like trachea, esophagus etc resulting in stridor, dysphagia etc. - Treated by rituximab/ tomoxifen/ steroids. EXTRA EDGE: - IgG4-Related diseases are: Mikulicz syndrome (enlargement and fibrosis of salivary & lacrimal glands) Riedel thyroiditis idiopathic retroperitoneal fibrosis autoimmune pancreatitis, and inflammatory pseudotumors of orbit, lungs & kidneys.