Riedel's Thyroiditis and Noninvasive Follicular Neoplasm With Papillary-Like Nuclear Features: A unique coexistence and diagnostic challenge.

Riedel's thyroiditis is a rare inflammatory disease characterized by extensive fibrosis, often mimicking malignancy. The concurrent presence of Riedel's thyroiditis with noninvasive follicular neoplasm with papillary-like nuclear features (NIFTP) is exceptionally rare and has not been previously reported. A 63-year-old male presented with an acute onset anterior cervical mass, fever, and dysphagia. Imaging revealed a large cystic mass in the right thyroid lobe extending into the mediastinum. Initial treatment with antibiotics led to partial improvement, but persistent swelling necessitated a deferred total thyroidectomy. Intraoperatively, the thyroid gland was fibrotic and adherent to surrounding structures. Histopathological examination confirmed Riedel's thyroiditis with extrathyroidal extension and an underlying NIFTP. Postoperatively, the patient developed left vocal cord paresis and primary hypoparathyroidism. Systemic fibrotic involvement was ruled out through a thoracoabdominal-pelvic computed tomography (CT) scan. This case highlights the diagnostic challenges, atypical presentation, and management strategies of this unique coexistence, initially masquerading as a thyroid abscess.