Riedel’s thyroiditis (RT) is a rare disorder characterized by chronic inflammation and extensive fibrosis of the thyroid gland. Clinically, it presents as a “stone-like,” “hard-as-wood,” non-tender goiter. The exact etiology remains unclear, but RT may represent an autoimmune thyroiditis, a systemic fibrosing disorder, or an IgG4-related disease. Because of its infiltrative nature, RT can mimic anaplastic thyroid carcinoma, thyroid lymphoma, or sarcoma, making diagnosis challenging.

A 65-year-old woman presented with progressive hoarseness and dyspnea for one month, accompanied by a rapidly enlarging anterior neck mass. Physical examination revealed a fixed, hard, non-tender thyroid without lymphadenopathy. Laboratory studies showed TSH 45.0 mIU/L and fT4 0.64 ng/dL; anti-TPO and anti-Tg antibodies were negative, and serum IgG4 was within normal limits. Ultrasound demonstrated a heterogeneous, hypovascular solid mass (6.3×3.5×3.9 cm) in the left thyroid lobe. Non-contrast CT revealed tracheal deviation and mediastinal extension, while PET scan showed intense FDG uptake. FNA suggested lymphocytic thyroiditis but could not exclude lymphoma. Open biopsy confirmed Riedel’s thyroiditis, showing dense fibrosis compressing thyroid follicles with lymphocytic infiltration. The patient was treated with levothyroxine and prednisolone, resulting in marked symptomatic improvement.

RT is rare, with an estimated incidence of 1.06 per 100,000 population, predominantly affecting women (3:1 ratio) aged 30–50 years. Imaging helps evaluate extrathyroidal invasion but cannot distinguish RT from malignancy. FNA provides limited diagnostic value; therefore, core or open biopsy remains crucial for definitive diagnosis. Corticosteroids are the mainstay of treatment, while tamoxifen and mycophenolate mofetil may serve as antifibrotic and anti-inflammatory agents. Minimal surgical intervention is recommended only for compressive symptoms.

Riedel’s thyroiditis is an uncommon but important differential diagnosis of hard thyroid lesions. Accurate diagnosis requires histopathological confirmation to prevent delayed recognition or inappropriate management.