Renal cell carcinoma metastatic to Hürthle cell adenoma of thyroid

Metastases to the thyroid gland are a common finding at autopsy in patients who died of malignancy and are often misdiagnosed as primary thyroid neoplasms clinically. We present a patient with a rare, unusual case of renal cell carcinoma (RCC) metastatic to a Hürthle cell adenoma of the thyroid. A 53-year-old woman was admitted to a University of Texas Medical Branch Hospital (Galveston, TX) for a large right thyroid mass that was present for 3 months. A fine needle aspiration of the thyroid mass was performed and interpreted as suggestive of a Hürthle cell neoplasm. A total thyroidectomy revealed Hürthle cell adenoma containing clusters of cytologically atypical cells with clear cytoplasm. Subsequent patient evaluation and computed tomography revealed a renal mass. Left radical nephrectomy was performed at a later date for left renal mass and the microscopic examination confirmed the diagnosis of primary clear cell carcinoma of the kidney. Further studies confirmed that the thyroid mass was metastases from RCC. Although carcinoma of the kidney is responsible in most instances of metastatic disease to the thyroid, metastatic RCC to a thyroid neoplasm is extremely rare, with only two reports found in the English literature. The possibility of metastatic RCC should always be taken under consideration, especially when nests of clear cells are seen infiltrating into the thyroid parenchyma or neoplasm.