Thyroid-Like Follicular Carcinoma of the Kidney.

Thyroid-like follicular carcinoma of the kidney (TLFCK) is a rare but emerging renal neoplasm that morphologically mimics follicular carcinoma of the thyroid but lacks immunohistochemical expression of thyroid markers such as TTF-1 and thyroglobulin. Here, a case of an incidentally discovered TLFCK in a 27-year-old man is reported. Histologic evaluation demonstrated an encapsulated proliferation of variably sized thyroid follicle-like epithelial-lined spaces filled with colloid-like eosinophilic secretions. Immunohistochemical analysis confirmed lack of expression of the thyroid markers TTF-1 and thyroglobulin with expression of PAX8 and CD10, confirming a neoplasm of renal origin, which correlated to the clinical and radiographic absence of thyroid pathology. In this report, this case is described with an emphasis on the differential diagnosis.