Thyroid tumors are usually associated with euthyroidism, and hyper functioning tumors are rare. There are few reports of carcinomas associated with hyperthyroidism. The aim of this study is to describe a case of hyper functioning papillary carcinoma associated with breast carcinoma. We present a 46-year-old woman that was referred for investigation of thyroid and breast nodules detected by routine ultrasound. She presented with hyperthyroidism and enlarged left thyroid lobe with fibroelastic consistency. Investigation demonstrated papillary carcinoma with follicular hyperactivity and invasive ductal carcinoma of the breast. The patient developed metastases 6 months later and died. This study demonstrates that hot thyroid nodules may harbor malignant tumors and should be punctured. The association with breast adenocarcinoma and the unfavorable outcome suggest higher aggressiveness of this tumor in the presence of hyperthyroidism.