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Background:Although very rare, carcinoid tumors of the lung may present as thyroid metastatic nodules, thus raising diagnostic difficulties for clinicians, surgeons, and pathologists. The most problematic differential diagnosis is with medullary thyroid carcinoma (MTC), a well-differentiated neuroendocrine tumor showing cytological and histological features similar to those of a lung carcinoid.Summary:We report a case of thyroid metastases from a typical carcinoid of the lung. The thyroid tumors were diagnosed in a 37-year-old woman known to have a lung nodule radiologically identified 8 years earlier, for which neither cytological nor histological analyses were performed. Fine-needle aspiration cytology of the thyroid nodule suggested a neuroendocrine tumor, possibly a MTC. However, the lack of both elevated calcitonin (CT) blood level and CT immunoreactivity in cytological smears ruled out the diagnosis of MTC in favor of a metastatic neuroendocrine tumor. Octreoscan demonstrated the neuroendocrine nature of the known lung neoplasm and the patient underwent right lung lobectomy together with total thyroidectomy. Histology examination confirmed the diagnosis of thyroid metastases from typical carcinoid of the lung. The patient is alive and free of disease 7 years after surgery.Conclusion:The rarity of this case offered the opportunity to discuss the most important criteria for distinguishing metastatic from primary neuroendocrine tumors of the thyroid and the indolent behavior of metastatic typical carcinoids of the lung. [ABSTRACT FROM AUTHOR]