IgG4-Related Retroperitoneal Fibrosis: A Rare Association With Riedel's Thyroiditis.

Idiopathic retroperitoneal fibrosis is a rare fibro-inflammatory disease that can be associated with other IgG4-related diseases (IgG4-RDs). It is exceedingly uncommon to encounter this condition in a patient with Riedel's thyroiditis (RT), another disease in the IgG4-RD family. We present the case of a 53-year-old man with a history of RT who presented for severe localized lower abdominal and suprapubic pain due to obstructive uropathy from extensive retroperitoneal fibrosis. The biopsy of the mass demonstrated fibro-inflammatory tissue, and its immunohistochemistry was notable for IgG4-positive plasma cells. This case highlights the challenge associated with the diagnosis and management of this rare manifestation of IgG4-RD. In a patient with a history of any form of IgG4-RDs, providers should be vigilant for any signs or symptoms that suggest the development of fibrosis in other organs.