Objective: Proinflammatory cytokines are known to affect the follicular epithelium in autoimmune thyroid disease. Here we investigated the effect of interferon-g (IFN-g) on the barrier function of primary cultured human thyrocytes. Design: Graves' thyroid follicle segments were cultured as a tight and polarised monolayer on the filter of a bicameral chamber, thereby allowing the in vivo epithelial characteristics to be maintained. Methods: Transepithelial electrical resistance was measured with a Millicell ERS ohmmeter. The tight junction proteins claudin-1 and occludin were analysed by immunofluorescence and Western blotting. Cell morphology was studied by transmission electron microscopy. Results: Thyrotrophin (TSH; 1 mU/ml) promoted the development of a tight epithelium monitored as a persistent increase in the transepithelial resistance to about 800 V·cm 2 . IFN-g (100 U/ml), on the other hand, decreased the resistance to 60-150 V·cm 2 after 48 h. In IFN-g-treated cells the expression of claudin-1, but not that of occludin, was decreased along with a diminished intracellular and cell surface immunostaining. In addition, claudin-1 was disrupted at cell-cell contacts. IFN-g also caused profound cell shape changes and a multilayered cellular organisation, without ultrastructural or biochemical (caspase-3 activity) signs of cytotoxicity. TSH was unable to counteract the effects of IFN-g. Conclusions: IFN-g destroys the barrier function of filter-cultured human thyroid epithelial cells. The loss of barrier involves down-regulation and an altered distribution of claudin-1. This novel effect of IFN-g on target cells in thyroid autoimmunity might be of pathophysiological relevance to the exposure of hidden autoantigens.