Riedel's Thyroiditis and Multifocal Fibrosclerosis are part of the IgG4‐related systemic disease spectrum

Objective Riedel's thyroiditis is a chronic fibrosing disorder of unknown etiology often associated with “multifocal fibrosclerosis.” IgG4‐related systemic disease is characterized by IgG4+ plasma cell infiltration and fibrosis throughout many organs. We hypothesized that Riedel's thyroiditis is part of the IgG4‐related systemic disease spectrum. Methods We searched our institution's pathology database using the terms “Riedel's,” “struma,” “thyroid,” and “fibrosis,” and identified 3 cases of Riedel's thyroiditis. Riedel's thyroiditis was diagnosed if there was a fibroinflammatory process involving all or a portion of the thyroid gland, with evidence of extension of the process into surrounding tissues. Immunohistochemical stains for IgG4 and IgG were performed. The histopathologic and immunohistochemical features of each involved organ were evaluated. The clinical features of one patient with multiple organ system disease were described. Results All 3 thyroidectomy samples stained positively for IgG4‐bearing plasma cells. One patient had extensive extrathyroidal involvement diagnostic of IgG4‐related systemic disease, including cholangitis, pseudotumors of both the lung and lacrimal gland, and a lymph node contiguous to the thyroid that stained intensely for IgG4+ plasma cells. The histologic features of all organs involved were consistent with IgG4‐related systemic disease. Patient 3 had 10 IgG4+ plasma cells per high‐power field initially, but rebiopsy 2 years later demonstrated no IgG4+ plasma cells. That patient's second biopsy, characterized by fibrosis and minimal residual inflammation, further solidifies the link between IgG4‐bearing plasma cells in tissue and the histologic evolution to Riedel's thyroiditis. Conclusion Riedel's thyroiditis is part of the IgG4‐related systemic disease spectrum. In many cases, multifocal fibrosclerosis and IgG4‐related systemic disease are probably the same entity.