It has long been known that there is a close relationship between pregnancy and the thyroid gland-iodine insufficiency, goitre development, and abnormal thyroid function are common during pregnancy. Autoimmune thyroid disease and thyrotoxicosis may also affect pregnant women, and their management requires the understanding of the interaction between the neuroendocrine and immune systems. In this article, the physiology of the thyroid gland during pregnancy and the interrelationship between pregnancy and autoimmune thyroid disease are discussed.