Thyroid Autoimmunity: An Interplay of Factors

The literature on thyroid autoimmunity has identified many potential factors at play for the initiation and progression of autoimmune thyroid diseases. These factors include genetic susceptibility, environmental factors, some drugs, iodine and selenium, infection, molecular mimics, and immune system defects. The sheer number of feasible factors makes sorting out the necessary agents from the fellow travelers difficult. In addition, many of these factors have the capability to interact—further confusing the picture. Another difficulty in interpreting these data is that most proposed mechanisms are not able to accomplish the triggering event in which the tolerance to self-antigens is actually overcome. In addition, some findings may be describing the conditions present after a disease is diagnosed and may be consequences of the disease rather than a cause. Recent description of the role of adipokines, which include leptin, tumor necrosis factor-alpha, and interleukin-6, in contributing to the inflammatory environment of the thyroid, along with the presence of thyroid Toll-like receptors for pathogen-associated patterns have the potential to deliver that necessary adjuvant signal to break tolerance, seen as necessary in animal autoimmune models. An additional factor, vitamin D3, due to its interaction both with white adipose tissue (WAT) and the immune system, has a complicated and somewhat controversial story with respect to thyroid autoimmunity. Conflicting results can result when not all factors are considered together. Aims: To describe the many factors at play in thyroid autoimmunity and how they interact. Conclusion: Thyroid autoimmunity is the result of an interplay of factors, with adipokines produced by WAT and vitamin D providing immune modulating signals external to the thyroid, while thyrocyte innate responses to environmental conditions provide the necessary adjuvant signal. Shaping the response to be reactive to particular self-antigens and likelihood of a response are due to genetics and molecular mimics.