A 52-year-old woman presents with a hard, fixed, woody thyroid mass adherent to surrounding neck structures. She has no lymphadenopathy. Biopsy shows dense, paucicellular hyalinized fibrosis with obliterative phlebitis, extending beyond the thyroid capsule into adjacent soft tissue. No malignant cells are seen. Which is the most likely diagnosis?

• A Anaplastic thyroid carcinoma
• B Reidel thyroiditis ✓
• C Hashimoto thyroiditis with fibrous variant
• D Primary thyroid lymphoma

Explanation

Reidel thyroiditis is a rare fibroinflammatory condition characterized by dense, paucicellular keloid-like fibrosis that infiltrates beyond the thyroid capsule into neck structures, with obliterative phlebitis. It can mimic malignancy clinically but lacks atypical cells. Anaplastic carcinoma would show pleomorphic malignant cells. Fibrous Hashimoto remains intrathyroidal and shows lymphoid infiltrate with Hurthle cells. Lymphoma shows diffuse lymphoid infiltrate without dense sclerosis.

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

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Written and medically reviewed by the StethoPrep medical team.