BACKGROUND: Thyroid metastasis from lung cancer is uncommon, accounting for approximately 8.3% of all thyroid metastases, which themselves occur in only about 1.4% to 3% of patients undergoing thyroid surgery; it poses significant diagnostic challenges due to its clinical and cytological resemblance to primary thyroid neoplasms.

CASE REPORT: We report the case of a 69-year-old woman with a history of non–small cell lung carcinoma (NSCLC) that had remained stable for 9 years, who presented with a progressively enlarging anterior neck mass. Ultrasound and fine-needle aspiration (FNA) initially suggested a benign thyroid lesion. However, given the marked interval growth of the nodule and the patient’s oncologic history, a right thyroid lobectomy was performed for diagnostic and therapeutic purposes. Histopathology supported by immunohistochemical analysis was consistent with the diagnosis of metastatic adenocarcinoma of pulmonary origin involving the thyroid. Imaging studies did not demonstrate additional metastatic sites, although a small pulmonary nodule of uncertain importance was noted. She was subsequently referred for systemic therapy. We analyze the diagnostic challenges, histopathologic characteristics, limitations of FNA, and the critical role of immunohistochemistry in differentiating metastatic lesions from primary thyroid carcinoma.

CONCLUSIONS: This case highlights the potential for misdiagnosis of metastatic thyroid lesions, particularly when cytological findings suggest a benign nodule, and underscores the importance of integrating clinical history, imaging, and immunohistochemistry to achieve an accurate diagnosis. Long-term surveillance remains essential in cancer survivors, as metastatic disease can occur after a prolonged disease-free interval.

Keywords: Immunohistochemistry, Thyroid Nodule, Carcinoma, Non-Small-Cell Lung