Approximately one-third of all pregnancies end in miscarriage. The etiology of recurrent abortion remains unknown in approximately 50% of all women. In the early 1990s it was discovered that unselected euthyroid women who present with thyroid antibodies (thyroid peroxidase and thyroglobulin) in the first trimester of pregnancy have a two–four-fold increase in their miscarriage rates. The majority of studies investigating women with recurrent abortion have also found a significant increase in thyroid antibody positivity compared with controls. Although the etiology of miscarriage in thyroid antibody women remains unknown, recent data have revealed a potential direct effect of thyroglobulin antibodies on pregnancy loss in a murine model. Uncontrolled studies assessing the effect of levothyroxine on decreasing the miscarriage rate in euthyroid antibody positive women, have demonstrated a decreased miscarriage rate.