Abstract: Metastasis of breast cancer to the thyroid is even rarer; clinical and imaging findings lack specificity, and definitive diagnosis relies primarily on pathology and immunohistochemistry.This report describes a 59-year-old woman who developed cervical lymph node metastasis following surgery for invasive ductal carcinoma of the left breast, and in whom bilateral thyroid nodules enlarged markedly within a short period during treatment with exemestane combined with dacarbazine.Total thyroidectomy and bilateral central lymph node dissection were performed.Postoperative pathology, combined with ER, PR, and GATA-3 positivity and PAX-2 negativity, confirmed thyroid metastasis from breast cancer.Therefore, in patients with a history of breast cancer-particularly those with hormone receptor-positive disease and a long disease course-metastasis should be suspected if rapidly growing thyroid nodules are detected during follow-up.

Key words: breast cancer; hormone receptor-positive; thyroid metastasis; metastatic thyroid tumor; case report; immunohistochemistry