The relation of thyroid autoimmunity to miscarriage is an important issue that has attracted the interest of many investigators. A number of papers have been published so far, which include healthy women, women with recurrent miscarriage and those undergoing assisted reproductive techniques. Most studies have shown a significant positive association between the presence of thyroid autoantibodies and miscarriage rate. It is of interest that women with high titers do not show a higher miscarriage rate when compared with women having low titers, although, there is no general agreement on this issue. There are three possible explanations for the assumed association of thyroid autoimmunity with miscarriage: 1) pregnancy loss is an epiphenomenon and not a direct effect of the thyroid autoantibodies, the presence of thyroid autoantibodies reflecting a generalized activation of the immune system; 2) delayed conception from the presence of thyroid autoantibodies; hence, when women with thyroid autoimmunity become pregnant, face a higher risk of miscarriage because of older age; and 3) the pregnancy loss is secondary to a subtle deficiency in thyroid hormone concentrations or a lower capacity of the thyroid to adequately adapt to the demands of pregnancy.