We present a retrospective case series of patients with hyperthyroidism and thyroid cancer. Our goal was to look at their clinical characteristics and outcomes to determine which patients would require further investigation. We reviewed the case notes of all patients with a histopathologic diagnosis of thyroid cancer and biochemical evidence of hyperthyroidism who had been treated at a thyroid cancer center from January 2006 through October 2013. During that time, 66 patients had been diagnosed with thyroid cancer. Of these, 8 patients (12%)-all women, aged 29 to 87 years (mean: 55.6; median: 50.5)-had biochemical evidence of hyperthyroidism. Among these 8 patients, 4 had an autonomously functioning toxic nodule (AFTN), 3 were diagnosed with Graves disease, and 1 had a toxic multinodular goiter. Five patients had suspicious features on preoperative ultrasonography. All 8 patients were diagnosed with the papillary type of thyroid carcinoma. The mean size of the tumor in the 4 patients with AFTN was significantly larger than it was in those with Graves disease (42.3 ± 23.8 mm vs. 3.8 ± 1.6; p = 0.04). The 3 patients with Graves disease all had incidentally found papillary microcarcinoma. Between these two groups, the patients with AFTN had a poorer prognosis; 2 of them had extracapsular invasion and lymph node metastasis, and another died of her disease. We found that the incidence of hyperthyroidism in thyroid cancer patients was relatively high (12%). In contrast to what has previously been reported in the literature, patients with AFTN seem to have more aggressive disease and poorer outcomes than do patients with Graves disease. Any suspicious nodule associated with hyperthyroidism should be evaluated carefully.