The uptake of iodide by thyroid cells requires the expression of sodium iodide symporter (NIS). Thyroid benign and malignant tumors have low iodide uptake activity. Previous studies of NIS expression with RT-PCR and immunohistochemistry showed divergent data. NIS protein was overexpressed in thyroid cancer. The aim of this study was to investigate the NIS transcript levels and its presence and localization in 30 samples of thyroid tumors (14 benign and 16 malignant) and in their surrounding non-tumoral tissues (NT), by real time RT-PCR and immunohistochemistry, respectively. Our results revealed lower gene expression in 78.6% of the benign tumors and 100% of the carcinomas when compared with the NT samples, using GHPDH as a housekeeping gene. Immunohistochemical staining revealed presence of NIS protein in 100% of the non-tumoral samples, 100% of the benign tumors and 93.75% of the malignant tumors. NIS protein was identified at basolateral membrane in 23.3% of non-tumoral samples, 14.3% of benign and 12.5% of malignant tumors. Stronger cytoplasmatic immunostaining of NIS protein was detected in 64.3% of benign tumors and in 87.5% of malignant tumors when compared to NT. Association between low gene expression and strong cytoplasmatic immunostaining was found in 50% of benign tumors and 87.5% of malignant tumors. We concluded that the reduced NIS gene expression in thyroid tumors associated with strong intracytoplasmatic staining may be due to its incapacity to migrate to cellular membrane.