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A 54-year-old woman presented with a palpable “grapelike” mass in the anterior neck. On ultrasound, the mass corresponded to a solid 1.5-cm lesion in the left lower thyroid lobe (Figure 1A). A fine needle aspiration (FNA) cytology procedure, performed under ultrasound guidance, showed overtly malignant cells with high-grade nuclei, relatively high nuclear to cytoplasmic ratio, and dense cytoplasm (Figure 1B; left: direct smear, Diff-Quik; right: cell block, hematoxylin-eosin). The diagnosis of “positive for malignant cells” was rendered. A left hemithyroidectomy was performed. Cut-section of the specimen showed a solitary circumscribed 1.5-cm nodule (Figure 1C). On microscopy, the tumor was nonencapsulated, populated by polygonal malignant cells, and centrally necrotic (Figures 1D-E). The malignant cells were similar to those observed in the previously performed FNA. In addition, minute intrathyroidal clusters of malignant cells (each <1 mm), similar to those in the main mass, were present in the vicinity of the dominant mass (arrow, in Figure 1D). The native thyroid parenchyma showed focal minimal nonspecific lymphocytic thyroiditis. The fortuitous revelation of the clinical history of left breast cancer—an invasive poorly differentiated ductal carcinoma diagnosed 3-month previously (Figure 1F)— prompted a comparative histopathological review. The carcinomas in thyroid and in the breast were strikingly similar. The tumor in the thyroid was negative for thyroid markers (TTF-1 and thyroglobulin) and positive for GATA3 (primarily a marker for tumors of breast and urothelial origin; inset in Figure 1E). Immunostain for PAX8 (a thyroid, Mullerian, and urinary tract marker) was negative. Furthermore, the carcinomas in the thyroid and in the breast were both triple-negative (ER: negative, PR: negative, and HER2: negative). The diagnosis of intrathyroidal metastasis (ITM) of breast carcinoma was rendered. 680746 IJSXXX10.1177/1066896916680746International Journal of Surgical PathologyIrshaid et al research-article2016