Metastatic struma ovarii is an extremely rare disease, and the treatment of choice has not been established. Here, we introduce the case of a 36-year-old female pregnant patient with metastatic struma ovarii. Initial treatment was an exploratory laparotomy to remove multiple peritoneal masses. After delivery, a total thyroidectomy was done for the further (131)I-therapy. (131)I-SPECT/CT and (18) F-FDG PET/CT showed multiple hepatic metastases and extensive peritoneal seeding nodules. Multiple (131)I and retinoic acid combination therapies were performed, resulting in marked improvement. (131)I-SPECT/CT and (18) F-FDG PET/CT were quite useful for evaluating the biologic characteristics of the metastases.