A 53-year-old woman presented with thyrotoxicosis, which is an unusual manifestation of thyroid carcinoma. Hyperthyroidism associated with malignancy usually occurs with well-differentiated follicular thyroid carcinoma. We show that extensive disease burden contributed to the development of hyperthyroidism, the occurrence of the Jod-Basedow phenomenon and the subsequent death of the patient. This diagnosis and treatment can be challenging.