Immune-endocrine interactions in autoimmune thyroid diseases

Autoimmune thyroid diseases (AITD) are the most common organ-specific autoimmune disorders affecting approximately 5% of the overall population. An aberrant interaction between abnormal thyrocytes, abnormal antigen-presenting cells and abnormal T cells forms the basis for the atypical autoimmune reaction targeting thyroid antigens. It was proposed that nongenetic (environmental and hormonal) factors play a crucial etiological role in AITD development, through altering immune-endocrine interactions. The most outstanding fact is that in genetically predisposed individuals, the disruption of these neuroendocrine-immune interactions by environmental factors results in thyroid autoimmune dysfunction. These interactions are able to incline the balance between Th1-Th2 immune response toward one side, resulting in a Th1-cell-mediated autoimmune reaction with thyrocyte destruction and hypothyroidism in Hashimoto's thyroiditis but to a hyperreactive Th2-mediated humoral response against TSH receptor with stimulatory antibodies leading to Graves' disease hyperthyroidism. In this review the main mechanisms involved are summarized. In this sense, the participation of stress-mediated activation of the sympathoadrenal system and hypothalamic-pituitary-adrenal axis, the hormonal changes occurring during pregnancy and postpartum acting on antigen-presenting cells and influencing, in this way, the balance of the immune status are shown to participate in AITD etiology. The possibility that altered levels of thyroid hormones during the course of the AITD may alter immune function is also discussed.