Thyroid diseases occur in more than 1% of the population and have been estimated to affect nearly twenty-seven million Americans. Thyroid diseases are observed 4- to 5- fold more frequently in women when compared with men, particularly during the childbearing period. Therefore thyroid function abnormalities are commonly encountered during a “routine” laboratory evaluation carried out for pregnant women. Iodine metabolism is intrinsically linked with thyroid metabolism and iodine is involved in the synthesis of thyroid hormones. Therefore, dietary deficiency of iodine commonly manifests in form of hypothyroidism. Correct diagnosis and treatment of thyroid dysfunction and underlying iodine deficiency during pregnancy is important to prevent both maternal and fetal complications. This would be primarily discussed in this chapter. However, during the course of pregnancy, normal physiological changes take place affecting the interpretation of normal tests assessing the efficacy of thyroid gland in the body. Thus before discussing the affects of iodine deficiency, the normal changes in thyroid function that occur during pregnancy and the interpretation of various laboratory tests used to diagnose thyroid disease during pregnancy would be discussed in this chapter. Though iodine deficiency disorders may not be prevalent in developed countries due to routine use of iodine supplemented common salt, these disorders are important in the developing part of the world particularly, South East Asia. In these areas in order to prevent neonatal cretinism and other complications, it is important to ensure that pregnant women take adequate supply of iodine in their diets, both before conception and in the antenatal period.