Medullary thyroid cancer arising from a thyroid rest: a case report

Medullary thyroid carcinoma (MTC) is rare and originates from parafollicular C cells and most cases present with a primary thyroid lesion. This report describes a 67-year-old woman with a left-sided neck mass and no evidence of intrathyroidal disease. Positron emission tomography-computed tomography (PET-CT) revealed paratracheal lymphadenopathy; other imaging and TSH were unremarkable. Fine-needle aspiration was inconclusive, and excisional biopsy suggested high-grade metastatic neuroendocrine carcinoma, initially suspected to be pulmonary due to thyroid transcription factor-1 (TTF-1) positivity. Pathology review raised concern for MTC. Endocrine evaluation showed elevated calcitonin (25.4 pg/ml) and carcinoembryonic antigen (CEA) (20.1 ng/ml). She denied personal or family history of thyroid disease or multiple endocrine neoplasia syndromes. Total thyroidectomy with central neck dissection was performed. All thyroid sections stained negative for calcitonin, excluding C cell hyperplasia or intrathyroidal MTC. Lymph node morphology and immunoprofile supported metastatic MTC arising from a thyroid rest. This is a rare entity, with only two other cases documented in the literature.