Thyroid hormone plays an important role in the process of implantation of a fertilized egg in the endometrium, as well as in the division and development of the fertilized egg. In addition, maternal thyroid hormone is essential for the formation and development of the fetal brain, nervous system, and bones. Therefore, when maternal thyroid hormone is insufficient, complications such as miscarriage, stillbirth, low birth weight and preeclampsia increase. Guidelines for the diagnosis and treatment of thyroid dysfunction during pregnancy were established and published by the American Endocrine Society, the American Thyroid Association, and the European Thyroid Society. The Korean Thyroid Society also published recommendations for diagnosis and treatment of thyroid disease during pregnancy and postpartum in 2014. However, since the revised guidelines of the American Thyroid Association were published in 2017, many studies, including large-scale randomized controlled trials, have been published in relation to thyroid and pregnancy, and the need to revise the existing recommendations has emerged in Korea. The main revisions of this recommendation are changes in the normal range of TSH during pregnancy, detailed description of treatment policies according to pregnant women’s autoantib...