SAT538 Thyrotoxicosis in the Setting of Metastatic Poorly Differentiated Thyroid Carcinoma, a Marker of Disease Progression?

Disclosure: B.L. Chang: None. M.A. Fernandez: None. K.M. Shikuma Lee: None. Background: Thyrotoxicosis is a rare manifestation of thyroid cancer, and poorly differentiated thyroid carcinoma (PDTC) causing thyrotoxicosis has even more rarely been reported. Here, we report a case of PDTC with thyrotoxicosis developing in a previously euthyroid individual with longstanding untreated follicular neoplasm. Clinical Case: A 79-year-old veteran presented to routine endocrine follow up with new symptoms of thyrotoxicosis. Although previously euthyroid, the evaluation revealed new findings of low serum thyrotropin (TSH) (<0.01 uIU/mL, n=0.34 – 5.60 uIU/mL), elevated free thyroxine (FT4) (2.6 ng/dL, n=0.58 – 1.64 ng/dL), and high total T3 (TT3) levels (223 ng/dL, n=76 – 181 ng/dL). Thyroid stimulating immunoglobulin antibodies (TSI) were positive (214, n<140). Patient has a nine-year history of a fine needle aspiration (FNA) of a large right-sided thyroid nodule with cytology suspicious for follicular neoplasm but declined surgical removal. Thyroid uptake and scan showed heterogeneous uptake throughout the right lobe with a relative photopenic region centrally and spots of more increased activity in areas within the right lobe. The left thyroid lobe showed uniformly low uptake. PET-CT evaluation confirmed spotty FDG-avid areas within the right thyroid mass but did not show other areas of hyperactivity. Methimazole was started with clinical and biochemical improvement. Patient underwent total thyroidectomy, and histopathologic examination showed PDTC with extensive angioinvasion involving the majority of the right thyroid lobe with the left lobe containing a mix of benign thyroid tissue and PDTC fragments. Areas of hyperintensity on the uptake scan correlated to pathological PDTC locations within the right lobe. Within three weeks of thyroidectomy, patient developed recurrent hyperthyroidism. Laboratory studies showed low TSH (0.04 uIU/mL) and elevated FT4 (2 ng/dL). In contrast to pre-operative studies, repeat TSI was below the reference range. Levothyroxine (LT4) was held in anticipation of radioactive iodine therapy, but four weeks later the TSH remained suppressed. (131)I was administered which showed accumulation within small thyroid remnants and a single left level three (131)I-avid lymph node without other metastases. Following (131)I treatment, patient returned to a euthyroid state on standard LT4. Conclusion: Hyperthyroidism has rarely been associated with PDTC, although previous reports have linked more aggressive, multifocal, and locally invasive thyroid cancers to concurrent Graves’ disease or productive bulky metastatic disease. Even without bulky metastatic disease, clinicians should be aware that hyperproductivity may indicate development of autoimmune thyrotoxicosis within PDTC. Presentation Date: Saturday, June 17, 2023