Objective: To describe a case of papillary carcinoma of the thyroid that mimicked a diffuse toxic goiter. Methods: We present a detailed case report, review related cases in the literature, and discuss the possible role of autoantibodies in promoting the growth of papillary thyroid tumor. Results: A 28-year-old woman had a 6-month history of weight loss, palpitations, increased anxiety, and enlargement of her thyroid gland. Laboratory studies confirmed the presence of hyperthyroidism. A 1-cm nodule was identified at the left upper pole of the thyroid. Fine-needle aspiration biopsy of this nodule revealed papillary carcinoma of the thyroid. Total thyroidectomy and a modified radical neck dissection were performed. Microscopic examination showed that the entire gland was replaced by a papillary carcinoma with a predominantly follicular pattern of growth. Of 44 lymph nodes tested, 31 were positive for metastatic papillary thyroid carcinoma. The presence of thyroid antibodies may have had a role in the aggressiveness of the patient's disease. Conclusion: To our knowledge, this is the first published report of a papillary thyroid cancer with manifestations resembling Graves' disease.