Metastasis to the thyroid from lung adenocarcinoma mimicking thyroid carcinoma

We herein report a case of metastasis to the thyroid from lung adenocarcinoma mimicking thyroid carcinoma. The thyroid tumor was palpated in the left lobe of the thyroid and diagnosed as primary thyroid carcinoma by fine-needle aspiration cytology. The patient also had a large pulmonary tumor and tiny pulmonary nodules, which were respectively diagnosed as moderately differentiated adenocarcinoma of the lung and intrapulmonary metastases from the main large lung carcinoma by the pathological examination of the biopsy specimens obtained by video-assisted thoracic surgery. Hemithyroidectomy with radical neck dissection was performed. The thyroid tumor was diagnosed as metastasis to the thyroid from lung adenocarcinoma, because it showed mucin production, positive immunoreactivity for carcinoembryonic antigen and negative immunoreactivities for thyroglobulin and calcitonin. The patient received systemic chemotherapy and died of the disease 1 year and 7 months after the diagnosis was made.