MON-345 A Unique Case of Functional Metastasis in Differentiated Thyroid Carcinoma Causing Thyrotoxicosis

Abstract Disclosure: R. Chien: None. A. Thakkar: None. Differentiated thyroid cancer is quite common, however it is rare to see metastatic differentiated thyroid cancer with functional thyroid tissue. Here, we describe a unique case of an 80 year-old female with PMH of CML, who developed functional thyroid cancer causing thyrotoxicosis, with discovery and diagnosis in both the US and Iran. The patient underwent a right, then partial left thyroidectomy in 1990 and 2012 for hyperthyroidism in Iran. She remained hyperthyroid post op and was started on methimazole (MMI). She was told she had thyroid cancer but did not need follow up. A CT chest in 2020 in the US revealed multiple solid pulmonary nodules, which were confirmed to be metastatic thyroid cancer after FNA biopsy in Iran. After the diagnosis, two 131I therapy attempts were made, however, as TSH was not high enough, ablation was not completed. The patient was referred to our endocrinology clinic for evaluation after surveillance imaging showed new and growing pulmonary nodules. We referred her for a completion thyroidectomy in 5/2023; pathology showed follicular thyroid tissue without evidence of metastasis. Thyroid hormone replacement was started post -op, however she became increasingly dyspneic, resulting in a hospitalization for atrial fibrillation and heart failure exacerbation from thyrotoxicosis. She was started back on MMI with resolution of her symptoms. Post surgical thyroglobulin (Tg) level peaked at 2725 ng/ml. The plan was to pursue 131I therapy after thyroidectomy, however therapy was postponed for treatment of newly diagnosed lung adenocarcinoma in situ and mucinous adenocarcinoma of the cecum. Finally, she proceeded with 131I therapy after a radioactive iodine scan with MMI withdrawal revealed iodine avid pulmonary and spinal metastasis, and was started on LT4. Tg 4 weeks after surgery dropped to 119 ng/ml. A PET/CT done 3 months later showed partial resolution of many of her lung nodules but unfortunately showed 2 new masses in the lung. They were biopsied and found to be adenocarcinoma of the lung. She decided to forego all further treatment at this point and moved back to Iran. She was offered genetic testing, however declined. Here we present an interesting case of an elderly patient with 4 primary malignancies including functional metastatic thyroid cancer. Thyrotoxicosis after thyroidectomy from functional thyroid metastasis is quite rare and this case demonstrates how it may be successfully treated with thyroidectomy and 131I therapy. These patients may need antithyroid drugs after thyroidectomy to control hyperthyroidism. Antithyroid drugs should be withdrawn prior to 131I therapy to allow adequate iodine uptake by the tumor. Presentation: Monday, July 14, 2025