The purpose of this study was to determine the expression of Na-bicarbonate-cotransporter NBCe1 in rat and human thyroid. It was demonstrated both at the mRNA and protein levels in thyroid using RT-PCR, immunohistofluorescence, and Western blotting.. Three variant specific primers were designed but only two NBCe1 variant transcripts (NBCe1-B and NBCe1-C but not NBCe1-A variants) were detected in rat and human thyroid. The localization of NBCe1 in rat and human thyroid was determined using a commercial antibody against a region common to the three NBCe1 variants. By immunofluorescence, only the basolateral membrane was labeled in rat thyrocytes, while both apical and basolateral membranes were immunostained in human thyroid. By western blot analysis on human thyroid primary cultured cells and rat thyroid cell line PC Cl3 cells, a ~130 kDa band was detected, and its expression increased with TSH or forskolin stimulation of both human primary culture cells and rat PC Cl3 cells. To test whether this also occurs in vivo, four groups of rat were studied for up to 4 weeks: 1) control rats, 2) oral treatment with methimazole (MMI) or 3) perchlorate (Per) and, 4) thyroxine (T₄), each of them given in the drinking water. The animals were sacrificed; their thyroïd gland was removed and used for immunocytochemistry and western blot analysis. Using these methods, we observed an up-regulation of NBCe1 in the stimulated thyroid of MMI and Per treated groups but a down-regulation in the resting thyroid of the T₄ treated group. Thus NBCe1 expression is positively regulated by TSH.