Thyroid Metastasis as the Initial Presentation of Occult Breast Carcinoma: A Case Report

Occult breast cancer (OBC) is a rare clinical entity characterized by metastatic disease in the absence of an identifiable primary breast lesion. Metastases to the thyroid gland are rare. Breast carcinoma infrequently metastasizes to the thyroid, and cases arising from OBC are exceptionally uncommon, posing a significant diagnostic challenge. We report the case of a 63-year-old woman with no prior history of malignancy who presented with diffuse abdominal pain, headache, and neck stiffness. A thyroid nodule was identified during diagnostic evaluation. Ultrasonography demonstrated a hypoechoic lesion with irregular margins, and core needle biopsy suggested malignancy. Immunohistochemical (IHC) analysis showed positivity for estrogen receptor (ER), progesterone receptor (PR), mammaglobin, and GATA3, with negativity for thyroglobulin (TG), PAX8, and thyroid transcription factor-1 (TTF-1), supporting a breast origin. Mammography and breast ultrasonography were negative. Breast magnetic resonance imaging (MRI) was not performed due to rapid clinical deterioration. During hospitalization, she developed a generalized tonic-clonic seizure. Imaging revealed multiple brain metastases, indicating advanced disease. This case underscores the diagnostic complexity of thyroid lesions and highlights the necessity of considering metastatic disease, particularly in the context of OBC. Accurate diagnosis requires comprehensive histopathological assessment supported by immunohistochemical profiling to reliably distinguish metastatic involvement from primary thyroid malignancies. Prompt recognition is crucial to inform optimal therapeutic decision-making in these challenging clinical scenarios.