We describe an unusual case of metastatic disease to the thyroid, characterized by massive intra-arterial embolization and clinical presentation as acute thyroiditis. The patient, a 37-year-old woman with a history of breast carcinoma, presented clinically with acute thyroiditis. No nodules were palpable, and fine-needle aspiration cytology of the left lobe was performed. It showed a pleomorphic carcinoma and was followed by total thyroidectomy. Histology disclosed a pleomorphic carcinoma diffusely affecting the thyroid gland. It was characterized by an extensive intra-arterial tumoral embolization. Immunohistochemistry confirmed the metastatic nature of the neoplasm. This is a very uncommon form of metastatic disease to the thyroid. The ischemia and necrosis associated with the embolic process were most probably responsible for this clinical presentation. This atypical clinical and cytologic presentation may induce confusion with a primary neoplasm, mainly anaplastic thyroid carcinoma.