Metastatic carcinoma

The breast, the kidney and the lungs are the most common primary sites of metastatic tumors to the thyroid. This possibility should be considered if the cytologic pattern does not conform with common neoplasms or the patient has a history of cancer elsewhere. In many cases, however, there is no clinical history of malignancy. Metastatic clear cell carcinoma of the kidney can be difficult to distinguish from a follicular or Hürthle cell neoplasm; the clinical history and immunocytochemical staining are helpful to make a correct a diagnosis. 

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