MON-331 Navigating the Uncommon- An Interesting Case of Malignant Struma Ovarii

Abstract Disclosure: L. Kandimalla: None. A. Jawahar: None. Struma ovarii is a monodermal teratoma that is predominantly composed of mature thyroid tissue and accounts for approximately 0.5-1% of all ovarian tumors. Majority of struma ovarii are benign and about 5-10% are malignant. We present a rare case of malignant struma ovarii that recurred several years after initial tumor resection. We had a 66 year old woman with past medical history of right ovarian cancer with malignant struma ovarii status post bilateral salpingo-oophorectomy who opted not to undergo total thyroidectomy at time of diagnosis. 14 years later, she was incidentally noted to have elevated liver enzymes, ALT 1575 U/L (range: 5-65) and AST 962 U/L (range: 5-40). CT imaging showed multiple peritoneal nodules suspicious for ovarian carcinoma. She was asymptomatic when she was initially seen in Endocrine clinic. Thyroglobulin at time of presentation was 580.5 ng/mL (range 1.5-38.5), Tg Ab <1 (range: 0-0.9), normal TSH and FT4. Whole body PET scan was negative for metastases and no radioactive iodine uptake was identified. Thyroid ultrasound showed a right sided 1.3 cm solid nodule, 2 left sided mixed cystic solid nodules. Total hysterectomy was performed when she was noted to have multiple retroperitoneal and intra-abdominal nodules that were excised and found to be consistent with metastatic papillary thyroid cancer, of the follicular variant. Following this, she underwent total thyroidectomy and pathology report showed benign thyroid tissue. She was then started on levothyroxine supplementation with a TSH suppression goal of <0.1. Thyroglobulin level post thyroidectomy was 8.6 ng/mL. Radioactive iodine scan showed new radioiodine avid lesions in the abdomen and pelvis most compatible with metastatic thyroid carcinoma. She was administered 200mCi I-131. Post treatment whole body scan showed radioiodine uptake localized to thyroid bed and previously demonstrated radioiodine avid abdominal lesions. Thyroglobulin level was 2.5 ng/mL 6 months post radioiodine treatment, with negative TgAb. CA-125 was normal before and after thyroidectomy. She is due for her follow up whole body PET scan. This case highlights the rarity and complexity of struma ovarii, particularly its unusual presentation and recurrence years after initial management. It emphasizes the need for further research to address unclear guidelines, such as whether thyroidectomy should be performed in these cases, followed by radioactive iodine 131 and suppressive thyroxine therapy. Presentation: Monday, July 14, 2025