The thyroid and autoimmunity.

Autoimmune thyroid diseases (Hashimoto thyroiditis, Graves' disease, postpartum thyroiditis, atrophic thyroiditis and drug induced thyroiditis) are prevalent disorders worldwide, especially in women (related to the millieu of sex steroids and X chromosome effects on the thyroid and the immune system). Disruption of thyroid self tolerance, usually induced by an infection, generates abnormal thyroid--immune interactions, implicating an array of cytokines and their receptors. Thyrocytes achieve antigen presenting cell properties which stimulate effector immune cells (Th1, Th2, Th17), in the context of defective immunomodulatory T regulatory cells, resulting in thyroid lymphocytic infiltration and activation of B cells, with production of antibodies against thyroid antigens, thyroid destruction or stimulation, depending on the Th1-Th2 balance. During pregnancy there is a Th2 predominance sustained by the increased glucocorticoid, estrogen and progesteron levels, which allows tolerance versus the histoincompatible fetoplacental unit. In the postpartum period, the return shift Th2 to Th1 favors the occurrence of postpartum thyroiditis. Altered thyroid hormone levels can influence the immune system, and, on the other side, some immune cells secrete TSH, which exerts endocrine and paracrine, cytokine-like effects. Understanding the complex pathogenesis of autoimmune thyroid disorders is crucial for prevention and management.