SUN-414 - A Case of Marine-Lenhart Syndrome: Grave'S Disease Stimulating New Onset Autonomous Hyperactivity in Preexisting Nonfunctioning Thyroid Nodules

Background
Marine-Lenhart syndrome is a rare form of hyperthyroidism defined by the presence of both Grave’s disease and autonomous hyperfunctioning thyroid nodules, occurring in around 0.26-1% of Grave’s disease patients (1,2). We present the diagnosis of this syndrome in a woman who developed Grave’s disease in association with new onset hyperactivity of her previously nonfunctioning thyroid nodules.

Clinical case
A 45-year-old woman was initially referred to endocrinology after an ultrasound performed for a palpated left neck bump revealed thyroid nodules. Formal ultrasound identified a solid isoechoic 2.1 cm TI-RADS 3 right thyroid nodule and a 3.9 cm mixed cystic and solid isoechoic TI-RADS 2 left thyroid nodule. Biochemical testing revealed normal TSH (0.94 mclU/mL, nl. 0.3-4.7 mclU/mL) and free T4 levels (1.3 ng/dL, nl. 0.80-1.70 ng/dL) consistent with nonfunctioning nodules. The patient underwent fine needle aspiration of the left thyroid nodule which demonstrated benign thyroid tissue with cystic change.

Two years later, the patient reported new-onset palpitations and heat intolerance. She was noted to have a suppressed TSH (0.02 mclU/mL), normal free T4 (1.60 ng/dL), elevated free T3 (429 pg/dL, nl. 222 - 383 pg/dL), positive thyroid stimulating immunoglobulin (2.87 IU/L, nl. < 0.54 IU/L), and positive TSH receptor antibodies (7.27 IU/L, nl. < 2.00 IU/L) consistent with Grave’s disease. A radioactive iodine uptake scan revealed an elevated uptake at 24 hours (32.5 %, nl. 15-30%). There was homogenous tracer uptake noted throughout the thyroid gland with two smaller areas of increased uptake in the left thyroid lobe representing two autonomous hot nodules. In combination, her Grave’s disease and autonomous hyperfunctioning thyroid nodules were consistent with a diagnosis of Marine-Lenhart syndrome.

The patient was offered treatment with methimazole as a bridge to surgery, surgery, or radioactive iodine therapy. Given a personal and family history of cancer, radioactive iodine was deferred. The patient elected for treatment initiation with methimazole (5 mg daily) for one year as a potential bridge to surgery. Repeat thyroid labs one month after revealed normalization of her TSH (0.46 mclU/mL), free T4 (1.20 ng/dL), and free T3 levels (299 pg/dL).

Conclusion
This case of the rare Marine-Lenhart syndrome illustrates the importance of investigating thyroid nodule functionality in patients with Grave’s disease and known thyroid nodules to ensure curative treatment. Treatment could otherwise potentially be complicated and prolonged.

References
1. Charkes ND. Grave’s disease with functioning nodules (Marine-Lenhart syndrome), J. Nuc. Med. 1972;13(12)885-892.
2. Danno H, Nishihara E, Kousaka K, Nakamura T, Kasahara T, Kudo T, Ito M, Fukata S, Nishikawa M, Miyauchi A. Prevalence and Treatment Outcomes of Marine-Lenhart Syndrome in Japan. A. Eur. Thyroid J. 2020;10(6):461-467.

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