SUN-312 From Hyperthyroidism to Malignancy: An Uncommon Presentation of Marine-Lenhart Syndrome

Abstract Disclosure: Y. Cho Yoon: None. I. Guatemala: None. Y. El Soufi: None. T. Zahedi: None. F. Zhang: None. Introduction: Grave’s disease is an autoimmune disease that leads to a generalized overactivity of the entire thyroid gland by activating the thyrotropin receptors of the thyroid cells which leads to increased synthesis and release of thyroid hormones. The coexistence of nodules can occur in up to 35% of patients and 0.8-2.7% of cases these nodules are functioning adenomas, which combination is known as Marine Lenhart syndrome. From all the nodules present in Marine Lenhart syndrome, the incidence of thyroid malignancy is approximately 10%. Here we present a rare case of Marine Lenhart syndrome with papillary thyroid carcinoma. Case description A 66-year-old female presented with 6-month history of unintentional weight loss, loss of appetite, heat intolerance, hair thinning, and palpitations. Labs revealed a TSH level of <0.015 µIU/mL and a free T4 (fT4) level of > 6.99 ng/dL, thyrotropin receptor antibodies (TRAb) at 11.70 IU/L (normal range <1.75 IU/L) and thyroid-stimulating immunoglobulin (TSI) at 8.37 IU/L (normal range <0.55 IU/L) which is indicative for Grave’s disease. Thyroid US showed numerous nodules with the largest one being 5 x 1.7 x 3.6 cm isoechoic, TIRADS 3. A thyroid uptake and scan was performed and the 24-hour iodine-131 uptake was calculated at 51%. On scintigraphy, the gland demonstrated multiple hot nodules in bilateral thyroid lobes suspicious for toxic multinodular goiter. Patient underwent right hemithyroidectomy. Pathology reported two separate foci of follicular variant papillary thyroid carcinoma with no vascular or extrathyroidal extension. Discussion: Marine-Lenhart syndrome is a rare condition characterized by the presence of hyperfunctioning thyroid nodules, often associated with Graves' disease. Typically, hyperfunctioning nodules identified as "hot" nodules are considered benign, and current guidelines recommend excluding them from fine-needle aspiration biopsy due to the extremely low risk of malignancy. The likelihood of carcinoma within a hot nodule is estimated to be less than 1%, reinforcing the general consensus that hyperfunctioning nodules can almost always rule out malignancy. However, this case report challenges the conventional understanding of hot nodules and their association with malignancy. While such cases are rare, they underscore the importance of considering atypical presentations and the potential limitations of existing guidelines. It also emphasizes the need for a tailored approach to diagnosis and treatment when hyperfunctioning nodules exhibit atypical features or associated pathologies. Presentation: Sunday, July 13, 2025