Radioactive iodine (RAI) therapy is used for the removal of remnant thyroid tissue or metastatic thyroid cancer cells in differentiated thyroid cancer. The main mechanism of the therapy is destruction of cells by radioactive iodine that penetrates the cells though the action of the sodium-iodide symporter (NIS). We experienced a case of a 26-year-old woman with mediastinal uptake as detected on a radioiodine scan, who was previously diagnosed with papillary thyroid cancer. For diagnostic tests including chest computed tomography (CT) and a radioiodine scan, the stimulated thyroglobulin level did not show a definite cause of the mediastinal uptake. During regular follow-up, the thymus became triangular with clear margins. The patient had neither specific symptoms nor physical findings related to the presence of a thymic mass. A subsequent CT scan showed an irregular margin of the thymus, suggestive of thymic metastasis. The patient underwent a mediastinectomy. The removed specimen was composed of normal thymic tissue. Moreover, we demonstrated the presence of human NIS by immunohistochemical analysis. After thymectomy, the mediastinal uptake was markedly decreased as compared to the previous scan. This case suggests that a clinician should be suspicious for the functional uptake of thymus when metastasis is unlikely in a clinical situation.