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.