Differentiated thyroid cancer sometimes shows microscopic vascular invasion but rarely causes a tumor thrombus in a great vein. A preoperative diagnosis of the latter condition is difficult if a patient does not have any overt symptoms, but it is important for operative planning. We encountered a 26-year-old woman with papillary thyroid cancer and a tumor thrombus extending from the brachiocephalic vein to the superior vena cava. The tumor was successfully treated by surgical resection. To our knowledge only 19 such cases have so far been reported. We herein report our case, review the literature, and discuss the associated diagnostic problems and treatments including such areas as age, gender, symptoms, pathology, extension, and outcome.