A 54-year-old woman who had undergone total abdominal hysterectomy and bilateral adnexial resection because of endometrial carcinosarcoma presented with thyrotoxicosis, rapidly enlarging neck mass, right cervical tenderness to the ears, palpitation, swelling and heat intolerance, 3 years after the operation. Her serum thyroid hormone levels confirmed hyperthyroidism. Triiodothyronine/thyroxine ratio was 18. The thyroid antibodies were markedly elevated. A thyroid scan showed a cold area corresponding to the thyroid nodule, with high radioiodine uptake in the remaining thyroid tissue. Antithyroid drugs were started to control the hyperthyroidism. In our patient, who had a history of malignancy, the rapidly enlarging neck mass was considered as metastasis to the thyroid gland. The patient underwent bilateral total thyroidectomy. Histopathological examination demonstrated findings compatible with Graves' disease on the left and a carcinosarcoma metastasis on the right thyroid lobe. We suggest that both destructive thyroiditis and Graves' disease contributed to our patient's thyrotoxicosis. Metastatic thyroid carcinoma and destructive thyroiditis should be considered in patients with rapidly enlarging neck mass and a prior history of visceral malignancy.