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Introduction Struma ovarii is a rare teratoma consisting of >50% thyroid tissue. Malignant transformation very seldomly occurs; most pathology shows papillary thyroid carcinoma. The risk and prevalence of concurrent thyroid gland neoplasm is unclear. Thyroid cancer surveillance post-oophorectomy in these patients may be pertinent but has not yet been established. Case Report A nulliparous 54 year-old woman with one year of menopause without HRT had pelvic and back pain. Imaging detected bilateral ovarian teratomas. Pathology from laparoscopic bilateral salpingo-oophorectomy revealed a 10.7×9.9×10.9cm noninvasive struma ovarii with focal papillary thyroid carcinoma of the left ovary and a benign teratoma of the right ovary. Importantly, the patient had a known thyroid nodule. Two years prior, a FNA of the 1.4cm hypoechoic, hypervascular nodule in the left lower pole yielded benign (Bethesda 2) cytology. After struma ovarii resection, thyroid ultrasound showed highly suspicious TIRAD5. She denied symptoms of hyper- or hypothyroidism and had a normal TSH. She consequently underwent two FNA biopsies, both of which were non-diagnostic. The patient deferred further attempts. History of an initial benign biopsy and stability of the nodule size provide some reassurance. With the patient's reluctance for surgery, serial thyroglobulin (Tg) and ultrasounds will be followed. Post-operative Tg was normal with negative Tg antibody. If Tg becomes markedly elevated, a thyroidectomy will be indicated. Discussion Given the TIRAD5 nodule, this patient's risk of thyroid malignancy is 60-70%. This is compounded with her setting of malignant struma ovarii, which may imply genetic predisposition for thyroid carcinogenesis. Since cytology could not be obtained on the two recent biopsies, definitive pathology by diagnostic lobectomy or thyroidectomy may be considered. A total thyroidectomy would provide the advantage of following Tg for cancer surveillance of both thyroid and ovarian malignancies. There is scant evidence on the treatment of malignant struma ovarii. A few small case series and a pooled review of 57 cases of low-risk (confined to the ovary) malignant struma ovarii in which 50 patients did not undergo thyroidectomy/I-131 ablation showed only a 7.5% recurrence rate at 25 years (Marti et al Thyroid 2012 Apr; 22 (4): 400-406). Hence, this study recommends reserving prophylactic thyroidectomy for patients with extra-ovarian extension or metastases. Treatment of malignant struma ovarii may be extrapolated from those used in differentiated thyroid cancer; high-risk lesions would be those measuring >4cm, have local or distant invasion, or have a BRAF mutation (indicating an aggressive tumor). In conclusion, conservative measures of the thyroid gland with a suspicious nodule may be acceptable in patients with noninvasive malignant struma ovarii, provided strict adherence to cancer surveillance of Tg with both thyroid and pelvic sonography. Presentation: No date and time listed