MON-363 - Protracted Hyperthyroidism after Total Thyroidectomy in Graves' Disease Revealing Functioning Struma Ovarii

The coexistence of Graves' disease and functioning struma ovarii is rare and can pose a diagnostic challenge. Persistent thyrotoxicosis after total thyroidectomy is most often attributed to residual ectopic thyroid tissue or levothyroxine excess, with ectopic thyroid tissue and struma ovarii being rare and potentially less-recognized causes. Thyrotropin receptor antibodies may induce hormone production from thyroid tissue existing outside of the neck, which is likely to escape detection on physical examination and standard thyroid imaging.

A 46-year-old woman with longstanding Graves' disease complicated by atrial fibrillation, cardiomyopathy (EF 40–45%), and prior thyroid storm underwent total thyroidectomy for difficult-to-control disease with ophthalmopathy. Preoperative evaluation supported classic Graves' disease, including markedly positive antibodies (TSI index 3.7 [normal ≤ 1.3], TRAb 24.00 IU/L [normal ≤ 1.75]) and thyroid ultrasound demonstrating a diffusely enlarged gland with heterogeneous echotexture. Surgical pathology showed diffuse hyperplasia. Postoperatively she was started on weight-based thyroid hormone replacement, but over the following 6 months she exhibited persistent thyrotoxicosis despite progressive reductions in levothyroxine dose, discontinuation of levothyroxine, and finally resumption of methimazole. Physical examination and neck ultrasound showed no evident thyroid tissue, and radioiodine uptake and scan of the neck showed minimal focal uptake in the thyroid bed (24-hour uptake 0.5%). Whole-body I-131 SPECT/CT revealed intense uptake in a 9.4 cm right pelvic mass. Transvaginal ultrasound confirmed the presence of an enlarged right ovary containing cystic mass-like areas. She underwent total hysterectomy and bilateral salpingo-oophorectomy, with pathology demonstrating a 13.7 cm mature teratoma with dominant thyroid tissue (>50%) showing hyperplastic and adenomatoid changes, without malignant features or vascular invasion, consistent with benign struma ovarii. Postoperatively she was restarted on levothyroxine 125 mcg daily, with subsequent normalization of free T4 and elevation of TSH (14.25 uIU/mL), consistent with dependence on thyroid hormone replacement.

Our case illustrates that persistent thyrotoxicosis following total thyroidectomy for Graves' disease should prompt consideration of remaining functional thyroid tissue, including struma ovarii. Minimal radioiodine uptake in the neck should raise concern for distant sites of thyroid hormone production, making whole-body I-131 SPECT/CT essential in this context. Surgical removal of the remaining thyroid tissue provides definitive therapy. Awareness of the rare combination of Graves disease and struma ovarii can prevent diagnostic misadventures and delays in curative treatment.

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