Thyroid hormone is essential for pregnancy and ensuring fetal development. Pregnancy also places substantial demands on the thyroid axis. Overt hypothyroidism is associated with substantial adverse obstetric and offspring outcomes and requires treatment. Borderline thyroid dysfunction is common in women and associated with adverse obstetric and offspring outcomes, although benefits of screening for and treating borderline thyroid function are unclear. Many women are established on thyroid hormone replacement before pregnancy and doses need increasing during pregnancy. Care is taken to prevent overreplacement. Universal thyroid screening in pregnancy is being undertaken in several countries, although it remains a matter of debate.