Hyperfunctioning metastatic follicular thyroid carcinoma in Pendred's syndrome. A 66-year-old woman with Pendred's syndrome underwent a partial thyroidectomy when she was 17 years old. At the age of 52 years, she had a second thyroid operation because of hyperthyroidism due to a toxic multinodular goiter with a mediastinal extension consisting of several separate nodules. Five years later a hyperfunctioning metastatic follicular carcinoma was diagnosed histologically. After treatment with radioactive iodine, the patient was well. To the authors' knowledge, this is the first description of a metastatic follicular thyroid carcinoma in Pendred's syndrome and the first report of hyperthyroidism occurring after malignant degeneration of a dyshormonogenetic goiter. 