Interleukin 2 (IL-2) is a lymphokine that may disrupt immunological self-tolerance. While being incapable of interfering with intrathymic or peripheral clonal deletion, IL-2 may overcome functional antigen unresponsiveness in anergic T lymphocytes. Anergy of T helper cells of the inflammatory phenotype implies selective silencing of the transcription of the IL-2 gene and thus precludes autocrine IL-2/IL-2 receptor (IL-2R) mediated growth, as well as delivery of help to other T cells or B lymphocytes. Thus, IL-2 serves as a servomodulator regulating post-deletional self-tolerance. IL-2-producing and IL-2-receptive cells are present in a variety of autoimmune lesions, including spontaneous autoimmune thyroiditis developing in the Obese strain (OS) of chickens, in Hashimoto's struma lymphomatosa, and in Graves' disease. Whereas the OS is characterized by a hyperinducibility of the IL-2/IL-2R system that predisposes to the development of severe thyroid infiltration, the state of the IL-2/IL-R system in circulating lymphocytes of patients developing thyroid autoimmunity, or at risk of doing so, remains to be defined. The most frequent autoimmune side-effect of IL-2 treatment concerns the thyroid gland. IL-2 induces a lymphoid thyroiditis leading to primary hypothyroidism, especially in those patients that have pre-treatment antithyroid autoantibodies. The hypothesis is extrapolated that IL-2 induces autoimmune disease in those patients that bear undeleted thyroid-specific T cells, and in which the lack of manifest thyroiditis relies upon peripheral, post-deletional tolerance.