THU661 Marine-Lenhart Syndrome: An Unusual Presentation Of Hyperthyroid Disease

Disclosure: B. Ji: None. O. Adeniran: None. L. Soni: None. Background: Marine-Lenhart syndrome, also known as nodular Graves’ disease, is defined as the coexistence of functioning thyroid nodules and Graves’ disease. It is a rare syndrome, with a prevalence of approximately 0.8-2.7%. The two diseases may coexist or present at different times [1]. Treatment options include high doses of antithyroid medications or radioactive iodine and thyroidectomy in selected cases [2]. Clinical Case: A 72-year-old female with a medical history of right-sided breast cancer treated with lumpectomy and radiation therapy in 2012 on anastrozole was referred for multinodular goiter (MNG) evaluation in 2018. She was clinically and biochemically euthyroid. Her thyroid ultrasound demonstrated two isoechoic right lobe nodules measuring 2.1cm and 1.3cm, respectively. An isoechoic rounded isthmus nodule measures 2.1cm. Three isoechoic nodules in the left lobe measuring 2.6cm, 2.2cm, and 1.7cm. She underwent fine needle aspiration (FNA) of two nodules in 2019, and both demonstrated benign follicular nodules with Bethesda II classification. At a follow-up visit in 2021, she reported unintentional weight loss but denied any other symptoms of hyperthyroidism, including heat intolerance, palpitation, tremor, or diarrhea. Physical examination was significant for exophthalmos, lid retraction, and diffusely enlarged thyroid with non-tenderness to palpation. Thyroid function tests revealed subclinical hyperthyroidism with TSH of <0.01uIU/mL (0.27-4.2uIU/ml) and FT4 of 1.5ng/dL(0.9-1.8ng/mL). Thyroid stimulating immunoglobulin (TSI) was 3.98IU/L (0.00-0.55IU/L), and TSH receptor antibody was 7.64 IU/L (0.00-1.75 IU/L). She was started on low-dose methimazole therapy. Thyroid uptake scan showed markedly elevated radioactive iodine uptake with 63.46% in 24 hours and focal nodular regions of increased uptake of the bilateral thyroid lobes compatible with toxic multinodular goiter. Repeated thyroid ultrasound in 2022 revealed stable-sized nodules with at least two new sub-centimeter nodules in both lobes. The patient was also seen by ophthalmology and plans to start teprotumumab treatment. At her most recent follow-up visit in January 2023, she has been continued on methimazole 2.5mg daily. Conclusion: The criteria for diagnosing Marine-Lenhart syndrome have not been well established. The possibility of Marine-Lenhart syndrome should be considered in hyperthyroid patients with positive antibodies and nodular goiter. In this case, she was initially asymptomatic with an enlarged multinodular goiter; three years later, she developed subclinical hyperthyroidism due to Graves’ disease and toxic multinodular goiter. Her symptom and thyroid function are well-controlled with low-dose methimazole. Presentation: Thursday, June 15, 2023