In the majority of the studies, SLC5A5 mRNA levels are lower in thyroid carcinomas than in adenomas (4) and normal adjacent thyroid (5, 6, 7); furthermore, SLC5A5 expression presents some limitations in predicting NIS expression and functionality: whereas a negative or low mRNA level may lead to reduced protein expression, a positive or high mRNA expression does not always correspond to higher protein levels or higher functionality (7, 8).