The course and severity of autoimmune thyroid disease are altered during pregnancy and in the postpartum period. The thyroidal response to a fluctuating immune status, combined with changes in thyroid economy during pregnancy, may result in a need to adjust the treatment regimen for thyroid disease during pregnancy. Patients with Hashimoto's hypothyroidism on thyroid hormone replacement are frequently observed to have an increased requirement for levothyroxine early in pregnancy, although this is not a universal finding. Hashimoto's hypothyroidism does not typically remit during pregnancy, although further progression of thyroiditis may be seen in the postpartum period. Graves' disease usually improves during pregnancy and flares after delivery, again necessitating monitoring of thyroid status and possible adjustments in thionamide therapy. However, spontaneous transformation from Hashimoto's hypothyroidism to Graves' disease during pregnancy is rare. We report a case of transient Graves' hyperthyroidism during the late second trimester in a patient on levothyroxine replacement for Hashimoto's hypothyroidism. This resulted in a need to discontinue the patient's thyroid hormone entirely to avoid exacerbation of her hyperthyroidism. This interesting case is presented, along with a discussion of how the expression of autoimmune thyroid disease may be altered during pregnancy.