ODP635 A rare case of metastatic papillary thyroid carcinoma on NTRK inhibitor with thyrotoxicosis

We present a case of 26-year-old Hispanic female with neck enlargement and worsening dyspnea. She was noted to have enlarged thyroid nodules; biopsy revealed papillary thyroid carcinoma (PTC) with ETV6-NTRK fusion by next-generation sequencing (NGS). Further work up revealed extensive bilateral milliary pulmonary metastases and Graves’ disease with elevated thyrotropin receptor antibodies and thyroid-stimulating immunoglobulin titers. Tuberculosis work up was negative and bronchoalveolar lavage revealed metastatic PTC. The patient was initiated on larotrectinib (100mg administered twice daily) as well as block-and-replace regimen with levothyroxine and methimazole. Her dyspnea improved and she successfully underwent total thyroidectomy as well as bilateral neck dissections; methimazole and thyroid hormones were discontinued thereafter. Interestingly, patient developed thyrotoxicosis post-surgery even without being on levothyroxine with elevated free T3 at 18 pg/mL (normal range 2.3–4.2), compared to 5.8 pg/mL prior to surgery. Despite discontinuation of levothyroxine, patient continued to be biochemically and clinically hyperthyroid. She was treated with methimazole for 4 months after surgery, and subsequently underwent radioactive iodine (RAI) ablation with 30 mCi of I-131 of sodium iodide. After RAI, thyroid function started to improve, prompting re-initiation of very low dose levothyroxine about 5 months after surgery. Since then, she has remained clinically stable. Presentation: Sunday, June 12, 2022 12:30 p.m. - 2:30 p.m.