Aspiration cytology of renal-cell carcinoma metastatic to the thyroid.

Fine needle aspiration (FNA) of a thyroid mass clinically suspected of being acute thyroiditis led to a cytologic diagnosis of hypernephroma metastatic to the thyroid and to the subsequent detection of the occult primary tumor. The FNA cytomorphologic findings were substantiated by cytochemical staining of FNA samples and confirmed by subsequent histopathologic examination of the resected thyroid. Postoperative studies revealed an expansive growth in the left kidney; analysis of the nephrectomy specimen showed an invasive renal-cell carcinoma. This case emphasizes the considerable value of FNA biopsy in making the frequently difficult preoperative differential diagnosis of primary and metastatic thyroid tumor and the importance of cytochemical analyses in making that distinction and in suggesting the site of the primary tumor.