A 50-year-old woman has a rock-hard, fixed thyroid mass causing stridor and hoarseness over six months. Intraoperatively the process infiltrates strap muscles and encases the trachea. Biopsy shows dense keloid-like fibrous tissue with sparse inflammation and only scattered residual follicles. Serum IgG4 levels are elevated. The most likely diagnosis is:

• A Anaplastic carcinoma
• B Riedel thyroiditis (IgG4-related sclerosing thyroiditis) ✓
• C Reidel variant of papillary carcinoma
• D Fibrosing variant of Hashimoto thyroiditis without systemic disease

Explanation

Riedel thyroiditis produces a densely fibrotic, woody-hard gland that extends beyond the capsule into neck soft tissues, mimicking malignancy clinically. It is now classified as an IgG4-related sclerosing disease, supported by elevated serum IgG4 and IgG4-positive plasma cells. Anaplastic carcinoma would show malignant pleomorphic cells on biopsy, and the absence of atypia here excludes it. Hashimoto lacks transcapsular fibrous extension and systemic IgG4 elevation.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.