Thyrotoxicosis due to functioning metastatic follicular thyroid carcinoma after twelve I-131 therapies.

We present a case of functioning metastatic follicular thyroid carcinoma (FTC) causing severe thyrotoxicosis despite four years 12 iodine-131 therapies (1.461 Ci cumulatively). Initially, the patient had ostalgia and fracture in the right femur. Surgery-confirmed metastatic bone FTC and thyroidectomy-confirmed FTC. One month later, iodine-131 treatment commenced. During the follow-up, different metastatic sites showed different outcomes. Lung metastases disappeared, a thigh metastasis persisted, a new metastasis in the head occurred and pelvic metastases deteriorated into a huge mass elevating thyroglobulin and causing thyrotoxicosis within 3 months. Presurgical PET/CT also demonstrated the massiveness of the pelvic metastases. Thyrotoxicosis disappeared after surgical removal of the pelvic lesion.