Thyroid replaced by fibrous tissue with inflammatory infiltrate.Fibrosis may extend to local structures (eg, trachea, esophagus), mimicking anaplastic carcinoma. ¹/₃ are hypothyroid.

Considered a manifestation of IgG₄-related systemic disease (eg, autoimmune pancreatitis, retroperitoneal fibrosis, noninfectious aortitis).

Findings: fixed, hard (rock-like), painless goiter.

Histology: Dense fibrous tissue extending beyond the tyroid capsule.