The strong genetic component is borne out in studies on monozygotic twins, with a concordance of 38–55%, with an even higher concordance of circulating thyroid antibodies not in relation to clinical presentation (up to 80% in monozygotic twins). Neither result was seen to a similar degree in dizygotic twins, offering strong favour for high genetic aetiology. Medications that influence thyroid function Certain medications or drugs have been associated with altering and interfering with thyroid function. Of these drugs, there are two main mechanisms of interference that they can have. One of the mechanisms of interference is when a drug alters thyroid hormone serum transfer proteins. Estrogen, tamoxifen, heroin, methadone, clofibrate, 5-flurouracile, mitotane, and perphenazine all increase thyroid binding globulin (TBG) concentration. Androgens, anabolic steroids such as danazol, glucocorticoids, and slow release nicotinic acid all decrease TBG concentrations. Furosemide, fenoflenac, mefenamic acid, salicylates, phenytoin, diazepam, sulphonylureas, free fatty acids, and heparin all interfere with thyroid hormone binding to TBG and/or transthyretin. The other mechanism that medications can utilize to interfere with thyroid function would be to alter extra-thryoidal metabolism of thyroid hormone. Propylthiouracil, glucocorticoids, propanolol, iondinated contrast agents, amiodarone, and clomipramine all inhibit conversion of T4 and T3. Phenobarbital, rifampin, phenytoin and carbamazepine all increase hepatic metabolism. Finally, cholestryamine, colestipol, aluminium hydroxide, ferrous sulphate, and sucralfate are all drugs that decrease T4 absorption or enhance excretion. HLA genes The first gene locus associated with autoimmune thyroid disease was major histocompatibility complex (MHC) region on chromosome 6p21. It encodes HLAs. Specific HLA alleles have a higher affinity to autoantigenic thyroidal peptides and can contribute to autoimmune thyroid disease development. Specifically, in<unk> ’s disease, aberrant expression of HLA II on thyrocytes has been demonstrated. They can present thyroid autoantigens and initiate autoimmune thyroid disease. Susceptibility alleles are not consistent in<unk> ’s disease. In Caucasians, various alleles are reported to be associated with the disease, including DR3, DR5 and DQ7. CTLA-4 genes This gene is the second major immune-regulatory gene related to autoimmune thyroid disease. CTLA-4 gene polymorphisms may contribute to the reduced inhibition of T-cell proliferation and increase susceptibility to autoimmune response. CTLA-4 is a major thyroid autoantibody susceptibility gene. A linkage of the CTLA-4 region to the presence of thyroid autoantibodies was demonstrated by a whole-genome linkage analysis. CTLA-4 was confirmed as the main locus for thyroid autoantibodies. Protein tyrosine phosphatase nonreceptor-type 22 gene PTPN22 is the most recently identified immune-regulatory gene associated with autoimmune thyroid disease. It is located on chromosome 1p13 and expressed in lymphocytes. It acts as a negative regulator of T-cell activation. Mutation in this gene is a risk factor for many autoimmune diseases. Weaker T-cell signaling may lead to impaired thymic deletion of autoreactive T cells, and increased PTPN22 function may result in inhibition of regulatory T cells, which protect against autoimmunity. Immune-related genes IFN-γ promotes cell-mediated cytotoxicity against thyroid mutations causing increased production of IFN-γ were associated with the severity of hypothyroidism. Severe hypothyroidism is associated with mutations leading to lower production of IL-4 (Th2 cytokine suppressing cell-mediated autoimmunity), lower secretion of TGF-β (inhibitor of cytokine production), and mutations of FoxP3, an essential regulatory factor for the Tregs development. Development of<unk> ’s disease was associated with mutation of the gene for TNF-α (stimulator of the IFN-γ production), causing its higher concentration. Preventable environmental factors, including high iodine intake, selenium deficiency, and infectious diseases and certain drugs, have been implicated in the development of autoimmune thyroid disease in genetically predisposed individuals. Iodine Excessive iodine intake is a well-established environmental factor for triggering thyroid autoimmunity. A higher prevalence of thyroid autoantibodies is in the areas with higher iodine supply. Several mechanisms by which iodine may promote thyroid autoimmunity have been proposed. Iodine exposure leads to higher iodination of thyroglobulin, increasing its immunogenicity by creating new iodine-containing epitopes or exposing cryptic epitopes. It may facilitate presentation by APC, enhance the binding affinity of the T-cell receptor, and activating specific T-cells. Iodine exposure has been shown to increase the level of reactive oxygen species. They enhance the expression of the intracellular adhesion molecule-1 on the thyroid follicular cells, which could attract the immunocompetent cells into the thyroid gland. Iodine is toxic to thyrocytes since highly reactive oxygen species may bind to membrane lipids and proteins. It causes thyrocyte damage and the release of autoantigens. Iodine also promotes follicular cell apoptosis and has an influence on immune cells (augmented maturation of dendritic cells, increased number of T cells, stimulated B-cell immunoglobulin production). Data from The Danish Investigation of Iodine Intake and Thyroid Disease shows that within two cohorts (males, females) with moderate and mild iodine deficiency, the levels of both thyroid peroxidase and thyroglobulin antibodies are higher in females, and prevalence rates of both antibodies increase with age. Sex Study of healthy Danish twins divided to three groups (monozygotic and dizygotic same sex, and opposite sex twin pairs) estimated that genetic contribution to thyroid peroxidase antibodies susceptibility was 61% in males and 72% in females, and contribution to thyroglobulin antibodies susceptibility was 39% in males and 75% in females. The high female predominance in thyroid autoimmunity may be associated with the X chromosome. It contains sex and immune-related genes responsible for immune tolerance. A higher incidence of thyroid autoimmunity was reported in patients with a higher rate of X chromosome monosomy in peripheral white blood cells. Another potential mechanism might be skewed X-chromosome inactivation, leading to the escape of X-linked self-antigens from presentation in the thymus and loss of T-cell tolerance. Having other autoimmune diseases is a risk factor for developing<unk> , and the opposite is also true. Autoimmune diseases most commonly associated to<unk>  include celiac disease, type 1 diabetes, vitiligo, and alopecia. The genes implicated vary in different ethnic groups and the incidence is increased in people with chromosomal disorders, including Turner, Down, and Klinefelter syndromes usually associated with autoantibodies against thyroglobulin and thyroperoxidase. Progressive depletion of these cells as the cytotoxic immune response leads to higher degrees of primary hypothyroidism, presenting with low T3/T4 levels, and compensatory elevations of TSH.
Hashimoto's thyroiditis
Graves' disease
Ord's thyroiditis
Nontoxic nodular goiter
De Quervain's thyroiditis
Riedel's thyroiditis
Autoimmune thyroiditis
Subacute thyroiditis