The priority groups for the prevention and treatment of iodine deficiency are women of childbearing age, pregnant women, breastfeeding women, infants and young children between the ages of 0 and 3, and preschool and school-age children. The iodine requirement of pregnant women is much higher than that of non-pregnant women, and the iodine ingested by pregnant women not only meets their own physiological needs, but also has to be supplied to the fetus to ensure the growth and development of the fetus. Iodine deficiency in the natural environment, pregnancy reaction of pregnant women, salt avoidance, etc. will lead to insufficient iodine intake of pregnant women. As the fetus grows, the need for iodine will increase. Since the iodine in the mother’s body has to supply both herself and the fetus, there is competition. Since the mother’s thyroid gland is functioning well and the fetus’s thyroid function is still in the developmental stage, the fetus is at a disadvantage in its ability to compete for iodine. Once the mother’s iodine intake is insufficient, it will lead to iodine deficiency in the fetus, resulting in brain development disorders, and after birth, the more severe symptoms will become cretinism, and the less severe ones will become subcretinism. The iodine supply for infants and young children comes mainly from breast milk, and the mammary gland has the function of concentrating iodine. At this time, the iodine intake of breastfeeding women needs to be supplied to themselves and their infants at the same time, which is also a sensitive group of people with iodine deficiency, and once iodine deficiency affects the growth and development of infants and young children. In addition, preschool and school-age children are in a period of rapid growth and development, the need for iodine increases significantly, and are very vulnerable to iodine deficiency. Therefore, women and children are the most vulnerable groups to iodine deficiency.