Ectopic calcitonin-secreting pulmonary neuroendocrine tumor mimicking medullary thyroid carcinoma

Calcitonin-secreting neuroendocrine neoplasms of the lung are extremely rare. Differentiating medullary thyroid carcinoma (MTC) from ectopic calcitonin secretion is challenging. Elevated calcitonin usually triggers a thyroid-focused workup, but ectopic secretion must be considered to avoid misdiagnosis and unnecessary thyroidectomy. We report a 54-year-old man presenting with weight loss and back pain. Serum calcitonin was markedly elevated at 1011 pg/mL (SI: 293 pmol/L) (reference range, <16 pg/mL [SI: <4.67 pmol/L] for basal levels in men). Thyroid ultrasonography showed a >2 cm nodule in the right thyroid lobe. Fine-needle aspiration cytology revealed atypia of undetermined significance, and calcitonin washout was negative. 18F-fluorodeoxyglucose positron emission tomography showed intense uptake in a 4.5 cm right lower lobe pulmonary mass and a 6.5 cm right adrenal mass, with no thyroid uptake. Histopathological analysis of the lung lesion confirmed high-grade pulmonary neuroendocrine carcinoma. Systemic chemotherapy was initiated, but the patient demonstrated disease progression and died from respiratory failure. This case illustrates how ectopic calcitonin production can mimic MTC. Comprehensive evaluation with functional imaging and immunohistochemistry is essential for accurate diagnosis. Multidisciplinary collaboration is crucial in distinguishing thyroid malignancy from ectopic hypercalcitoninemia.