MON-329 Metastatic Thyroid Involvement In Her2-positive Breast Cancer Mimicking Primary Thyroid

Abstract Disclosure: D. Saba: None. L. Li: None. S. Katsnelson: None. J.A. Vu: None. A 51-year-old female with a history of human epidermal growth factor receptor 2 (HER2) -positive invasive ductal carcinoma (IDC) was found to have a right lobe nodule and a left thyroid lobe mass with high uptake on positron emission tomography (PET) scan. While fine-needle biopsy of the left thyroid lobe mass was inconclusive, biopsy of a right lobe nodule was suspicious for breast cancer. Both lesions resolved following systemic HER2-targeted chemotherapy, indicating metastatic breast cancer in the thyroid gland. This case highlights the diagnostic challenge of distinguishing primary thyroid malignancy from metastatic disease and the efficacy of HER2-targeted therapies in treating breast cancer metastases to the thyroid gland. Presentation: Monday, July 14, 2025