Four disorders of the postpartum period are associated with thyroid dysfunction. The most common is PPT. Although recovery from thyroid dysfunction often occurs in PPT, many patients eventually develop permanent hypothyroidism. Postpartum Graves' Disease is less common than PPT, but it is not unusual. Whereas antithyroid drugs are indicated for postpartum Graves' Disease, they are not useful in PPT. Although they are rare, lymphocytic hypophysitis and postpartum pituitary infarction are important entities because they cause deficiencies of many critical hormones. The autoimmune nature of PPT, postpartum Graves' disease, and lymphocytic hypophysitis highlights the unique effects of pregnancy on the immune system.