There is a growing consensus that autoimmune thyroid diseases, similar to other autoimmune diseases, is multifactorial: both several genetic and environmental factors interact and produce the clinical phenotype of these disorders. Twin studies and familial aggregation, including clustering within families showed that they are complex diseases with a significant genetic component. Several genetic factors associated with autoimmune thyroid diseases susceptibility have been identified, including the HLA genes, cytotoxic T lymphocyte associated-4 (CTLA-4) gene, TSH receptor and other immunoregulatory genes. Regarding environmental factors, although multiple factors including infection, stress, sex steroids, pregnancy, aging and food, are known as factors precipitating autoimmune thyroid diseases, little progress has been achieved defining them. It will be paradoxically important to identify genetic factors to investigate environmental factors.