Tehran University of Medical Sciences Case Reports in Clinical Practice 2538-2683 10 6 2026 06 03 260 264 Fariba Binesh Department of Pathology, Shahid Sadoughi University of Medical Sciences, Yazd, Iran. AND Hematology and Oncology Research Center, Shahid Sadoughi University of Medical Sciences, Yazd, Iran. Najmeh Beheshti Department of Pathology, Shahid Sadoughi University of Medical Sciences, Yazd, Iran. Fatemeh Khaleghi Dehshiri Department of Radiology, Shahid Sadoughi University of Medical Sciences, Yazd, Iran. FatemeAlsadat Aghaei Meybodi Department of Pulmonology, Shahid Sadoughi University of Medical Sciences, Yazd, Iran. Maryam Vajihinejad Department of pathology,Shahid Sadoughi university of medical sciences,Yazd,Iran 2026 02 20 2026 04 26 Pulmonary small cell carcinoma typically metastasizes to lymph nodes, liver, adrenal glands, brain, and pleura; thyroid involvement is exceptionally rare. A 57-yearold woman presented with an eight-month history of progressive dyspnea. Clinical examination revealed an enlarged thyroid gland with palpable nodules in both lobes. Contrast-enhanced chest CT demonstrated a lobulated right hilar mass extending into the posterior mediastinum. CT-guided biopsy of the hilar lesion confirmed small cell lung carcinoma on histopathology. Fine needle aspiration of the thyroid nodules yielded hypercellular smears containing medium-sized round cells with hyperchromatic nuclei. To exclude primary medullary thyroid carcinoma with pulmonary metastasis, serum calcitonin and carcinoembryonic antigen (CEA) were measured, revealing levels of 20 pg/mL and 49.91 ng/mL, respectively. Integrating these findings, the diagnosis of pulmonary small cell carcinoma with thyroid metastasis was established. This case underscores that, although rare, the thyroid gland can serve as a metastatic site for small cell lung carcinoma. Therefore, in patients with a known primary malignancy, the appearance of a new thyroid nodule warrants prompt evaluation for possible metastatic disease. https://crcp.tums.ac.ir/index.php/crcp/article/view/1172 https://crcp.tums.ac.ir/index.php/crcp/article/download/1172/759