Abstract
Thyroid hormone plays a critical role in fetal growth and development, and thyroid dysfunction during pregnancy is associated with several adverse outcomes, such as miscarriage and preterm birth. In this review, we introduce and explain three major changes in the revised Korean Thyroid Association (KTA) guidelines for the diagnosis and management of thyroid disease during pregnancy: first, the normal range of thyroid-stimulating hormone (TSH) during pregnancy; second, the treatment of subclinical hypothyroidism; and third, the management of euthyroid pregnant women with positive thyroid autoantibodies. The revised KTA guidelines adopt 4.0 mIU/L as the upper limit of TSH in the first trimester. A TSH level between 4.0 and 10.0 mIU/L, combined with free thyroxine (T4) within the normal range, is defined as subclinical hypothyroidism, and a TSH level over 10 mIU/L is defined as overt hypothyroidism regardless of the free T4 level. Levothyroxine treatment is recommended when the TSH level is higher than 4 mIU/L in subclinical hypothyroidism, regardless of thyroid peroxidase antibody positivity. However, thyroid hormone therapy to prevent miscarriage is not recommended in thyroid autoantibody-positive women with normal thyroid function.