We report two cases of thyrotoxicosis-revealing functional metastases of a follicular carcinoma that extended to the bones, liver and kidneys in one case and to the lungs in the other. Both patients had undergone surgical intervention for a thyroid nodule more than 15 years before the diagnosis of thyrotoxicosis and metastatic dissemination. In both the cases, the carcinoma was not recognized by the pathologist after the first surgical intervention, but was finally diagnosed several years later due to the occurrence of thyrotoxicosis. Iodine-131 therapy was effective at suppressing the thyrotoxicosis in both the patients. The effectiveness on the metastatic extension was very different for each patient: in the first case, the patient died a few years later without any control of the metastatic tissue. For the second patient, the metastases disappeared a few months after radioiodine treatment, with the patient still in remission more than 10 years later. The physiopathology and the evolution of these two cases are discussed with the data available in the literature.