Transformation of Hashimoto’s hypothyroidism to Graves’ hyperthyroidism is extremely rare, but does occur. The course and severity of autoimmune thyroid diseases are altered during pregnancy and the postpartum period. We report the clinical course, laboratory findings, and treatment of a female patient with 4 Hashimoto hypothyroidism. She had a past medical history of Hashimoto hypothyroidism and had been on LT replacement therapy for two years. Six-months postpartum, however, she developed Graves’ hyperthyroidism. 4 LT was discontinued, and the patient was started on methimazole, 30 mg daily. Methimazole was discontinued one month later due to severe side effects, and 131 radioiodine therapy was applied. We suggest that patients with preexisting Hashimoto’s hypothyroidism are not immune to developing Graves’ disease. When such patients have unexpected symptoms and changes in thyroid function tests, especially during pregnancy or in the postpartum state, this unlikely diagnosis should be entertained and treated accordingly.