Metastasis to the thyroid gland is rare in clinical settings despite its rich vascular supply. A 60-year-old woman who had rectal cancer was diagnosed with thyroid malignancy with bilateral cervical lymph node metastases and primary adenocarcinoma of the left upper lung. We compared findings of H&E and various immunohistochemical stains including Thyroglobulin, CK7, CK20, CEA, TTF-1 of specimens of thyroid tumor, lung cancer and rectal cancer after total thyroidectomy. Thus, we achieved the final diagnosis of thyroid tumor as metastasis to the thyroid glands from primary adenocarcinoma of the lungs. In patients with a history of extra-thyroidal adenocarcinomas, we should consider metastasis to the thyroid gland. Immunohistochemical staining including thyroglobulin, CK7, CK20, CEA, TTF1 might be helpful to differentiate between primary thyroid carcinoma and metastasis from extra-thyroidal carcinoma to identify their origin.