Genetic factors play an important role in the pathogenesis of autoimmune thyroid 
disease (AITD) and it has been calculated that 80% of the susceptibility to 
develop Graves' disease is attributable to genes. The concordance rate for AITD 
among monozygotic twins is, however, well below 1 and environmental factors thus 
must play an important role. We have attempted to carry out a comprehensive 
review of all the environmental and hormonal risk factors thought to bring about 
AITD in genetically predisposed individuals. Low birth weight, iodine excess and 
deficiency, selenium deficiency, parity, oral contraceptive use, reproductive 
span, fetal microchimerism, stress, seasonal variation, allergy, smoking, 
radiation damage to the thyroid gland, viral and bacterial infections all play a 
role in the development of autoimmune thyroid disorders. The use of certain 
drugs (lithium, interferon-alpha, Campath-1H) also increases the risk of the 
development of autoimmunity against the thyroid gland. Further research is 
warranted into the importance of fetal microchimerism and of viral infections 
capable of mounting an endogenous interferon-alpha response.