An 85-year-old man who had undergone a right hemicolectomy for colon cancer presented with severe hypothyroidism and hoarseness 21 months after the operation. The serum thyrotropin (TSH) was markedly elevated to 118.14 microIU/mL and serum free thyroxine (fT4) level was markedly suppressed to 0.34 ng/dL. Symptoms of hoarseness and neck swelling were already evident 4 months prior at which time tests for normal thyroid function were performed. The patient was referred due to aggravated pain on his diffusely enlarged hard goiter. An enlarged thyroid with some calcification was noticed in the neck ultrasonography with multiple cervical lymphadenopathies. Core biopsy of the thyroid gland showed invasion of poorly differentiated adenocarcinoma cells. Immunohistochemical studies showed positive staining only for carcinoembryonic antigen (CEA). There were multiple lung parenchymal nodules and adrenal masses at the time of evaluation. The patient was started on palliative chemotherapy with thyroid hormone replacement and gradually became euthyroid. From these findings and the clinical observations, thyroid metastasis with hypothyroidism developing acutely from metastatic colon adenocarcinoma was diagnosed.