Lipopolysaccharide (LPS) is an endotoxin from Gram-negative bacteria that exerts a wide variety of biological responses and is present in blood during certain infectious processes. Thyroid function and specific gene expression are regulated by thyrotropin (TSH). The first step in thyroid hormonogenesis is the NIS-mediated iodide uptake. NIS has been proposed as a possible autoantigen in autoimmune thyroid disease. The aim of this work was to analyse the effect of LPS on iodide uptake and NIS gene expression in TSH treated FRTL-5 cells. We observed that iodide uptake was stimulated by LPS (maximum 0.1µg/ml, 48h). LPS increased NIS protein level (immunofluorescence/confocal microscopy) (maximum 0.1µg/ml, 48h). The NIS mRNA level was increased by LPS (RT-PCR). In transient transfections assays, LPS increased NIS promoter (-2841 to +13 bp) activity in a concentration and time-dependent manner (18-24h). The expression of the LPS receptor, TLR4 was evidenced (RT-PCR). In conclusion, LPS was able to stimulate NIS gene expression at transcriptional level. The increment of the NIS level could explain the increase of iodide uptake produced by LPS. A novel property of thyroid cell to express TLR4 was revealed. These findings support a potential role of LPS in thyroid pathophysiology.