A Case Report of Riedel's Thyroiditis Behaving as Anaplastic Thyroid Carcinoma: A Rare Presentation.

This case report describes a 47-year-old male with a history of hypothyroidism, presenting with a rapidly enlarging neck mass, dysphagia, and dyspnea, raising clinical suspicion for anaplastic thyroid carcinoma (ATC). Initial investigations, including post-contrast CT and fine-needle aspiration cytology (FNAC), were inconclusive but suggested a possible malignant etiology. A total thyroidectomy was planned to relieve airway compression and rule out malignancy. Intraoperative findings revealed diffuse fibrosis and infiltration into surrounding structures, limiting surgical resection. Only a localized isthmectomy was performed. Histopathological examination with further immunohistochemical staining (IHS) confirmed the diagnosis of Riedel's thyroiditis (RT), a rare condition that mimicked ATC both clinically and radiologically. Postoperatively, the patient was treated with high-dose corticosteroids followed by tamoxifen, leading to significant clinical improvement and near-complete resolution of the thyroid mass within one year. This case underscores the importance of considering RT in the differential diagnosis of rapidly enlarging thyroid masses to avoid unnecessary, aggressive treatment.