The thyroid gland plays a major part in the human body; it’s in charge of creating the hormones necessary for providing our body’s energy levels for an active life. These hormones have a critical impact on the development and growth of our body. At the same time, however, the thyroid is highly vulnerable to autoimmune thyroid diseases (AITDs).
Why is the thyroid gland vulnerable to autoimmune thyroid disease?
Autoimmune thyroid diseases often occur as a result of genetic factors. These seem to affect disease progression after the thyroid gland becomes the target of autoimmunity cells. It can create a large array of cytokines, adhesion molecules, and also growth factors. The susceptibility of the thyroid to AITDs can come from the complexity of hormonal synthesis, oligoelement conditions and particular capabilities of the defense mechanisms of the thyroid.
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Genetic Factors for Thyroid Disease
The thyroid gland is essential in the human body due to it’s production of thyroid hormones. These compounds have fundamental effects, playing critical roles in somatic growth, brain development, bone maturation, and the mRNA synthesis of more than 100 proteins which govern each and every physiological function of the body.
The incidence of chronic autoimmune thyroiditis, abbreviated as CAT, and Graves’ disease, another autoimmune thyroid disease, has increased dramatically over the last few decades, afflicting up to 5 percent of the overall population. Chronic autoimmune thyroiditis has become the most common cause of hypothyroidism, according to recent studies.
Initial research on the association between early fetal nourishment and the pathogenesis of autoimmune thyroid disease has led to some controversial data. In twin studies, Phillips et al. found that one monozygotic twins, the smaller twin, had greater levels of thyroid peroxidase (TPO) antibodies. In instances of both type 1 and type 2 diabetes, the hypothesis suggests excess weight and obesity in children can be a major contributing factor to thyroid disease, even among the population of children with type 1 diabetes, those who were more overweight or obese demonstrated a higher prevalence of thyroid and autoimmunity issues. Obese children have also been found to have increased interferon (IFN)-γ-secreting T helper cells along with altered thyroid structure and hormonal status.
Recent advances in genome-wide research have made it feasible to effectively identify complicated genes. Using both the candidate gene approach and whole-genome linkage studies, 6 AITD susceptibility genes have been identified and verified; the very first group includes the immunomodulatory gene products HLA-DR, CD40, cytotoxic T lymphocyte-associated variable (CTLA-4), and protein tyrosine phosphatase 22 (PTPN22), and the second category involves the thyroid-specific receptor goods thyroglobulin (Tg) and also thyroid-stimulating hormone receptor (TSHR).
Genetic factors predominate as the leading cause of thyroid dysfunction, accounting for roughly 80 percent of the odds of developing autoimmune thyroid diseases, whereas at least 20 percent are due to environmental or other variables. On the history of AITDs, a number of research studies have been published in the last few decades.
An increased frequency of AITDs is reported in Turner syndrome and in additional nondisjunctional chromosomal disorders such as in Down syndrome and Klinefelter syndrome. The concept that autoimmunity can lead to the survival of a fetus with chromosomal aneuploidy is fascinating but remains unproven. The most prevalent disorder in Turner syndrome appears to be chronic autoimmune thyroiditis, with a reported thyroid autoantibody incidence of 30. Hypothyroidism of autoimmune origin is indeed common in TS that every other TS girl will likely develop hypothyroidism.
Analyzing the Thyroid Cell
The thyroid cell produces a variety of immunologically active factors, as shown on Table 1, and has complicated nutrient conditions for hormonal synthesis, as shown on Table 2, both of which influence susceptibility to AITDs. It is increasingly clear that the target thyroid cells interact with the body in ways that look defensive and protective, yet they can malfunction and exacerbate under circumstances that are particular, according to the research.
In the majority of human autoimmune disorders, the events which trigger autoimmunity remain unknown. It is unclear whether results mostly from an immune defect, are secondary to target organ alterations, or even both. The thyroid gland demonstrated increased oxidation and iodine uptake prior to infiltration concomitant in the case of autoimmune thyroid disease. Modifying thyroid function influences the growth of thyroid autoimmunity. The thyroid, contrary to other cells from the endocrine system, is exceptional in function since it releases hormonal goods on its basal surface instead of its surface, hence allowing for the proper transportation of iodine which is essential across the cell.
Thyroid cells are capable of producing different factors (Table 1), such as IGF I, IGF II, and EGF, which could stimulate angiogenesis. The half-life of those molecules is short and they induce only localized (nonsystemic) effects. Thyroid follicular cells secrete several necessary growth factors. The expression of intercellular adhesion molecule-1 (ICAM-1) and lymphocyte function-associated antigen-3 (LFA-3) by thyroid cells is enhanced by IFN-γ, tumor necrosis factor (TNF), and interleukin (IL)-1. Thyroid cells express CD44, which functions as a homing receptor for hyaluronan, mediates leukocyte rolling (the very first step in tissue homing), and may (like ICAM-1) induce lymphocyte activation under certain circumstances. Thyroid cells are now known to produce many cytokines (particularly after stimulation with IL-1), including IL-1, IL-6, IL-8, IL-12, IL-13, and IL-15. Activated lymphocytes can create TSH.
Low dose substance tolerance can easily be altered, and the thyroid gland is not well taken by the immune system. Autoantigens in autoimmune thyroid disease, as in other autoimmune endocrine disorders, include tissue-specific membrane receptors, enzymes, and secreted hormones. Mixed cellular and antibody autoimmune responses are probably pathogenic to some level. Circulating anti-Tg autoantibodies can also be present in GD and CAT, as are autoantibodies to triiodothyronine (T3) and thyroxine (T4). The human (h) TSHR is the principal antigenic target in autoimmune hyperthyroidism. The TPO autoantibody appears to be unlikely to have much pathogenic importance as it’s restricted access to TPO is due to its location inside the cell. What’s more, anti-TPO autoantibodies don’t inhibit the action of this enzyme. Therefore, their clinical significance is principally to record thyroid gland autoimmunity.
In humans, excess thyroid gland hormones can lead to the attenuation of natural killer (NK) cell activity, which in theory could cause the continuation of an autoimmune disease. Upon return to some euthyroid status along with the consequent normalization of NK activity, a reversion to management of this abnormal immune reaction would occur with perpetuation of GD. Furthermore, an anti-idiotype may be the agonist for the original antigen. Therefore, an antibody to a antibody (anti-idiotype) into TSH may pertain to the TSHR and stimulate the thyroid gland. A more likely hypothesis is that anti-idiotypic antibodies are rarely generated at a detectable degree. Hodkinson et al. recently found a positive association between thyroid hormone concentration and NK-like T cells in the elderly.
Bottazzo et al. first indicated that antigen presentation by HLA-DR-expressing thyroid gland hormones may be a vital aspect of thyroid autoimmune disease. It rapidly became clear in the research that the only stimulus capable of causing MHC class II expression on thyroid cells was the T cell cytokine IFN-γ. Regular cells react precisely the same as AITD thyroid gland in IFN-γ, and in animal models of AITDs, class II expression on thyroid cells is constantly followed by the amount of lymphocytes. Along with inducing MHC class II expression, IFN-γ increases MHC class I expression on thyroid cells, thus allowing the recognition of thyroid cells from cytotoxic CD8+ T cells.
It is likely that direct antigen presentation by the thyroid may occur in individuals who inherit T cells; such a circumstance would skip the standard mechanism. The HLA-DR antigen-expressing thyroid cell may be as successful as the macrophage at introducing thyroid-specific antigens to the immune system, but the thyroid cell is incapable of supplying the costimulatory signals that practitioner antigen-presenting cells (APCs) do. Any stimulus that causes increased DR expression on thyrocytes, for example IFN-γ produced by T cells in response to infection, combined with increased TSH stimulation may allow thyrocytes to function properly. Though thyroid cells can perform this function they are localized, allowing for production of the previously established occurring low levels of antibodies, leading to autoimmune thyroid disorder in the human body.
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By Dr. Alex Jimenez
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