The incidence of thyroid autoantibodies in women with recurrent fetal loss, infertility or women who miscarried appears to be increased compared with controls of reproductive age without previous abortions. There are few working hypotheses concerning the asociation between anti-thyroid antibodies and the increased risk for pregnancy loss. The first hypothesis suggests that women with high titers of anti thyroid antibodies have underlying very mild thyroid “under function”. Another theory views the anti-thyroid antibodies as simply secondary markers of a predisposition of autoimmune disease rather than the actual cause of pregnancy loss. An evolutionary explanation suggests that reproductive problems in women with high titers of autoantibodies exist in order to prevent the transmission of autoimmune genes to the next generation. The reason for the association between pregnancy loss and thyroid immunity is still not clear. The working hypotheses above supply multi factorial explanations which could act together, may even be in synergy, as the propulsion for the pregnancy loss. Until today the mechanism by which anti Tg are responsible for pregnancy loss is not clear. Induced animal models with high titier of anti Tg could provide direct evidence for the pathogenic role of anti-thyroid antibodies and the mechanism that is responsible for the pregnancy loss in autoimmune thyroiditis. In the current presentation we describe data concerning the association between anti thyroid antibodies and pregnancy loss, and hypothesis which explain this association.