Metastasis of breast cancer to the thyroid gland.

Metastases to the thyroid gland from nonthyroidal malignant tumors are rare but significant. They are often asymptomatic, indicating advanced-stage primary tumors and poor prognosis. Although infrequently, breast cancer (BC) can metastasize to the thyroid gland. We present the case of a 56-year-old woman with a history of BC who presented with a nodular goiter. Physical examination and imaging revealed a thyroid nodule and cervical lymph nodes with suspicious features. Fine-needle aspiration biopsy (FNAB) confirmed the presence of atypical epithelial cells in the thyroid nodule and lymph nodes. Further evaluation, including positron emission tomography, histological biopsy, and immunohistochemistry, supported the diagnosis of metastatic BC to the thyroid gland. Due to the local extent of the disease, total thyroidectomy was not feasible. The treatment with ribociclib and letrozole was initiated, but unfortunately, the patient had an unfavorable progression with the development of metastasis in the nervous system. Metastatic carcinoma to the thyroid gland is rare but has increased due to improved diagnostic techniques. BC can metastasize to the thyroid. Diagnosis involves imaging, FNAB, and immunohistochemistry. Treatment options include surgery, radiotherapy, and chemotherapy, but the prognosis is generally poor.