Incidental Thyroid Neuroendocrine Carcinoma Revealing a Presumed Occult Gastrointestinal Malignancy

In rare cases, a thyroid mass may represent metastatic neuroendocrine disease rather than a primary thyroid malignancy. This case report describes a unique presentation of neuroendocrine carcinoma involving the thyroid with concurrent somatostatin receptor-positive gastrointestinal and mesenteric lesions of uncertain origin, providing insight into the diagnostic and management challenges of this disease. We present a 60-year-old male patient found to have an incidental 11 cm left thyroid mass with airway displacement on neck computed tomography (CT). A left thyroid lobectomy was performed, and surgical pathology confirmed neuroendocrine carcinoma. Immunohistochemical and molecular testing argued against medullary thyroid carcinoma, while site-of-origin markers created a diagnostic challenge with features suggestive of both pulmonary and non-pulmonary origin. Subsequent DOTATATE positron emission tomography (PET) scan demonstrated somatostatin receptor-avid lesions involving the duodenum and mesentery, raising the possibility of a gastrointestinal primary with metastatic spread to the thyroid. The discordance between the immunohistochemical profile and imaging findings highlights the diagnostic complexity of neuroendocrine tumors involving the thyroid and underscores the importance of comprehensive systemic evaluation, including functional imaging and site-of-origin immunohistochemistry, in the setting of atypical thyroid pathology. Prompt recognition of potential systemic neuroendocrine malignancy is essential for staging and therapeutic planning.