A 55-year-old woman presents with a hard, fixed thyroid mass and hoarseness. Histology shows dense, keloid-like fibrosis extensively infiltrating beyond the thyroid capsule into surrounding skeletal muscle, with obliterative phlebitis. No malignant cells are seen. The diagnosis is:

• A Anaplastic thyroid carcinoma
• B Riedel thyroiditis ✓
• C Hashimoto thyroiditis
• D Reidel-like variant of papillary thyroid carcinoma

Explanation

Riedel thyroiditis is a rare fibrosing disorder showing dense, keloid-like fibrosis that extends beyond the thyroid capsule into surrounding structures including skeletal muscle, with obliterative phlebitis. It can mimic malignancy clinically but lacks malignant cells. Anaplastic carcinoma would show pleomorphic malignant cells. Hashimoto thyroiditis shows lymphocytic infiltrate without dense transmural fibrosis invading muscle. The absence of malignant cells distinguishes it from carcinoma.

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.