Risk factors
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.
Autoimmune diseases most commonly associated to Hashimotos thyroiditis 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. Pathophysiology
Multiple  mechanisms by which the pathology of Hashimotos thyroiditis develops have been suggested. Various autoantibodies may be present against thyroid peroxidase, thyroglobulin and TSH receptors, although a small percentage of people may have none of these antibodies present. As indicated in various twin studies, a percentage of the population may also have these antibodies without developing Hashimotos thyroiditis. Nevertheless, antibody-dependent, cell-mediated cytotoxicity is a substantial factor behind the apoptotic fall-out of Hashimotos thyroiditis. Activation of cytotoxic T-lymphocytes (CD8+ T-cells) in response to cell-mediated immune response affected by helper T-lymphocytes (CD4+ T-cells) is central to thyrocyte destruction. As is characteristic of type IV hypersensitivities, recruitment of macrophages is another effect of the helper T-lymphocyte activation, with Th1 axis lymphocytes producing inflammatory cytokines within thyroid tissue to further macrophage activation and migration into the thyroid gland for direct effect.Gross morphological changes within the thyroid are seen in the general enlargement, which is far more locally nodular and irregular than more diffuse patterns (such as that of hyperthyroidism).