Autoimmune thyroid disease is the most common organ-specific autoimmune disease and is a very common cause of thyroid dysfunction such as autoimmune hypothyroidism, Graves' disease and postpartum thyroiditis. The thyroid gland from patients with autoimmune thyroid disease is morphologically characterized by massive infiltration of lymphoid cells. The interleukin-1 (IL-1) family of molecules is together with other cytokines an integral component of the complex intercellular communication required to mount and control an immune response. IL-1alpha/beta in moderate and high concentrations reversibly inhibit thyroid cell function, while IL-1beta in low concentrations stimulates thyroid cell function. The biphasic, non-cytotoxic and reversible influence of IL-1 supports a role of IL-1 in the physiological regulation of thyroid cell function. IL-1 stimulates the guanylate mediated pathways and inhibits the adenylate cyclase mediated pathways. All IL-1 effects are counteracted by IL-1 receptor antagonist indicating that the effects are exerted through activation of specific IL-1 receptors on thyrocytes. Furthermore, IL-1 induces or enhances expression of a number of immunologically active molecules such as adhesion molecules, cytokines, and complement regulatory proteins in thyroid epithelial cells. IL-1 may thus play a role during physiological as well as pathophysiological conditions contributing to for example the euthyroid sick syndrome and development of thyroid autoimmunity. This review summarizes current literature on the phenomenological in vitro influence of IL-1 on the thyroid cell.