In normal thyroid tissues, NIS immunohistochemical expression was mainly localized in the basolateral plasma membrane of follicular cells. NIS positivity was detected in a few foci of isolated follicles throughout the tissue and within the positive follicles most of the cells were positive. Positivity was more frequently detected in small follicles composed by cuboidal and columnar cells and rarely detected in large follicles limited by flattened cells (Fig. 3A). In Graves’ disease, NIS was widely expressed and present in the basolateral plasma membrane of the great majority of follicular cells (Fig. 3B). In carcinomas, NIS staining was observed in 71.6% of the cases (74.8% of cPTCs, 69.8% of fvPTCs, 80.9% of other PTC variants, 55% of FTC and 67% of PDTC). Its location was predominantly in the cytoplasm (124/211) (Fig. 3C) and nucleus (15/211) and only 12/211 of the cases presented NIS in the basolateral plasma membrane of tumor cells (Fig. 3D). Figure 3NIS immunoexpression in different thyroid tissues. (A) Normal thyroid; (B) Graves’ disease; (C) cytoplasmatic staining in an oncocytic PTC; (D) membrane staining in a fvPTC; (E and F) NIS immunoexpression in a fvPTC without and with TSA amplification signal, respectively; (G and H) NIS immunoexpression in a FTC without and with TSA amplification signal, respectively; (I) negative staining in a cPTC and strong membrane staining in the surrounding Graves’ disease. In F and H, notice the loss of cytoplasmatic staining after the use of TSA amplification system. Bar 100 μm.