Clinically detectable metastatic follicular thyroid carcinoma to the thymus is very rare in the literature and sometimes confused with false positive uptake of Iodine¹³¹ (I¹³¹) in the normal thymus or hilar lymph node. The authors report a 35-year-old woman with metastatic follicular carcinoma to the thymus. She underwent total thyroidectomy and I¹³¹ ablation with 200 mCi. Six months later, a follow-up I¹³¹ whole body scan showed continued radioactive iodine uptake in the retrosternal area and the serum thyroglobulin level remained continuously elevated with levothyroxine suppression therapy (22.3 and 36.4 ng/ml, 6 and 10 months after total thyroidectomy, respectively). CT scan of the chest revealed several aggregated cystic lesions in the retrosternal mediastinum, suspected to represent mediastinal metastasis. The surgically resected retrosternal mass was confirmed as a metastatic follicular carcinoma to the thymus. After surgical excision, the serum thyroglobulin level was below 1.0 ng/ml and I¹³¹ whole body scan showed no radioactive iodine uptake in the mediastinum. The patient showed no evidence of recurrence after excision of thymic metastasis during one year of 0.2 mg levothyroxine suppression therapy. We report a case of metastatic follicular thyroid carcinoma to the thymus without bone or pulmonary involvement.