SUN-368 Atrial Fibrillation Following an Ovarian Mass Resection Unveiling Struma Ovarii-Induced Hyperthyroidism

Abstract Disclosure: N. Cazac: None. G.I. Sydney: None. L. Rosoph: None. V. Cazac: None. J.J. Wysolmerski: None. Background: Hyperthyroidism affects up to 1.4% of the population, with Graves’ disease and autonomous thyroid nodules the most frequent causes. These conditions are characterized by excessive thyroid hormone synthesis and secretion by the thyroid gland. Rarely, hyperthyroidism arises from ectopic production of thyroid hormones. Struma ovarii is a monodermal ovarian teratoma defined as containing more than 50% thyroid tissue, and it accounts for less than 1% of all ovarian tumors. Hyperthyroidism occurs in only 5-10% of struma ovarii cases, often due to excessive local hormone production. Clinical Case: A 65-year-old female underwent elective resection of a 29 cm complex abdominopelvic mass. In the early postoperative period, she developed atrial fibrillation (AFib) with rapid ventricular response. As part of AFib workup, thyroid function tests (TFTs) were obtained, revealing overt hyperthyroidism, with TSH suppressed to 0.126 μIU/mL (N: 0.27 - 4.2 μIU/mL) and free T4 elevated to 2.26 ng/dL (N: 0.8 - 1.7 ng/dL). Despite these biochemical findings, the patient exhibited no clinical signs of thyrotoxicosis. Thyroid autoantibodies (including thyroid peroxidase antibodies, TSH receptor antibodies, and thyroid stimulating immunoglobulin) were negative, and thyroid ultrasound demonstrated normal vascularity with no suspicious thyroid nodules. Surgical pathology of the mass confirmed a benign struma ovarii with diffuse thyroglobulin staining. The patient was initiated on beta-blockers for adrenergic symptom management, yet antithyroid drugs (ATDs) were deferred given resection of the hormone-producing tumor. Repeat TFTs one week postoperatively showed normalization of TSH and a decreasing free T4 level. Conclusion: Surgical resection is the recommended treatment for hyperfunctioning struma ovarii, as it addresses hyperthyroidism and allows for pathological evaluation of the ovarian mass to rule out ovarian malignancy. Current guidelines emphasize the importance of restoring a euthyroid state with ATDs and adrenergic blockade in cases of overt hyperthyroidism prior to surgery. However, standardized protocols for managing asymptomatic patients with potential hyperfunctioning struma ovarii are lacking. This case highlights the importance of vigilance when evaluating ovarian tumors, as hormonally active struma ovarii can complicate the surgical course and recovery. Awareness and tailored management are critical to prevent life-threatening complications, such as thyroid storm, particularly in the perioperative setting. Presentation: Sunday, July 13, 2025