Autoimmune thyroid disorders (AITDs) are the result of a complex interplay between genetic and environmental factors, the former account for about 70-80% of liability to develop AITDs. However, at least 20-30% is contributed by environmental factors, which include certainly smoking (at least for Graves' disease and orbitopathy), probably stress, iodine and selenium intake, several drugs, irradiation, pollutants, viral and bacterial infections, allergy, pregnancy, and post-partum. Evidence for the intervention of these factors is often limited, and the mechanisms whereby environmental factors may concur to the onset of AITDs are in many instances unclear. Nevertheless, gene-environment interaction seems a fundamental process for the occurrence of AITDs.