Locally produced proinflammatory cytokines are likely to play a pathophysiological role in autoimmune thyroid disease. An important feature of the thyroid, not previously considered in cytokine actions, is the barrier created by the follicular epithelium, which secludes two lumenal autoantigens [thyroglobulin (Tg) and thyroperoxidase] from the extrafollicular space. We examined the influence of recombinant cytokines on the barrier function of human thyrocytes cultured as a tight and polarized monolayer in bicameral chambers. Whereas interleukin (IL)-6 (100 U/mL), interferon-gamma (100 U/mL), tumor necrosis factor-alpha (10 ng/mL), and transforming growth factor-beta1 (10 ng/mL) had no effects, exposure to IL-1alpha for 24-48 h reduced the transepithelial resistance from >1000 to <50 omega x cm2 and increased the paracellular flux of [3H]inulin and exogenous 125I-Tg. This response to IL-1alpha, which was dose dependent (1-1000 U/mL) and reversible, was accompanied by dramatic morphological changes of the epithelial junction complex, including aberrant localization of the tight junction protein zonula occludens-1. At the same time, IL-1alpha decreased the apical secretion of endogenous Tg and stimulated the basolateral release of a novel high-molecular-mass protein. We conclude that IL-1alpha reduces the thyroid epithelial barrier without signs of general cytotoxicity. The observation suggests a mechanism by which IL-1alpha may promote the exposure of hidden autoantigens to the immune system in thyroid autoimmunity.