12214 Metastatic Functional Thyroid Carcinoma As An Unusual Cause Of Hyperthyroidism

Abstract Disclosure: S. Jimenez Salazar: None. N. Aristizabal Henao: None. C. Aguilar Londoño: None. N. Buitrago Gómez: None. S. Saldarriaga Betancur: None. D.L. Beltran: None. J.L. Torres: None. Background: Differentiated thyroid carcinoma (DTC) represents the most prevalent endocrine neoplasm. Less than 10% present distant metastases, with the lung (50%), and skeleton (25%) being the most frequently affected sites. The location of bone metastases predominates in the spine and pelvis; usually as osteolytic lesions, being more frequent in follicular thyroid cancer (FTC) (7-28%) than in patients with papillary thyroid cancer (PTC) (1.4-7%). In rare cases this metastasis can be functional and trigger a thyrotoxicosis syndrome with or without recombinant human TSH injections prior to therapy with 131-I. Clinical case: 59-year-old woman with a history of bone metastatic follicular thyroid cancer (FTC) presented with 1 month of palpitations, 4 kg weight loss, non-dysenteric diarrhea and progressive dyspnea. Her FTC had been diagnosed 8 years prior her consultation. In that time iodine refractoriness was considered due to the progression of bone lesions, and Sorafenib 800 mg per day was initiated, which was not tolerated due to severe dermatological manifestations, and the patient decided not to continue with TKI therapy. Since then, she has been treated with zoledronic acid and local radiotherapy as part of palliative care. On physical examination, she was tachycardic (150 bpm), and hypertensive (BP 160/100 mmHg). A suppressed TSH (0.01 uUI/mL, n: 0.4-4 uUI/mL) with elevation of thyroid hormones (free T4 3.99 ng/dL, n: 0.8-2 ng/dL and total T3 480 ng/dL, n: 45-140 ng/dL) was documented, associated with a patient who had required a dose adjustment of levothyroxine from 700 mcg to 350 mcg weekly 2 months prior to admission. Additionally, she had a significantly elevated thyroglobulin (5000 ng/mL, n: 0.1-64.1 ng/mL) with undetectable antithyroglobulin antibodies (1.09 UI/mL, n: 0-4.11 UI/mL). Considering the history of total thyroidectomy and the high tumor burden, secondary thyrotoxicosis due to functioning bone metastases of DTC was considered. Levothyroxine was discontinued, and propranolol was initiated for the control of adrenergic symptoms, along with methimazole, cholestyramine, and systemic steroids to control thyroid hormone production. 3 days after therapy adjustment dyspnea, palpitations, and thyroid function tests improved significantly (free T4 1.43 ng/dL, n: 0.8-2 ng/dL and total T3 178 ng/dL, n: 45-140 ng/dL). A new administration of therapy with 131-I 200 mCi was indicated. Antithyroid therapy and beta-blockers were continued with sustained biochemical control. Conclusion: Functional bone metastases of FTC can lead to high morbidity and mortality. Treatment consists of surgical cytoreduction of any large accessible metastatic lesion, therapy with 131-I, and high-dose antithyroid drugs to control hyperthyroidism. It is important to avoid the use of recombinant human TSH due to the risk of secondary thyrotoxicosis following its administration. Presentation: 6/2/2024