Malignant struma ovarii is a very rare tumor, with considerable disagreement concerning the necessary histologic features for malignancy. The prognosis with patients with a malignant struma ovarii is difficult to make because of inadequate follow-up of the reported cases and long clinical courses. In most cases the patients responded well to surgical treatment but sometimes patients have died from malignant struma ovarii, in particular, if there is metastasis. There is evidence that radioactive iodine is effective in treating metastatic struma. The present case reports the use of intraperitoneal chronic phosphate for metastatic intraperitoneal disease, with thyroid suppression. Long-term follow-up will be necessary to properly evaluate this therapy. Prophylactic administration of thyroid hormone should be considered in cases of malignant struma ovarii.