Due to the lack of direct evidence of optimal iodine intake of Chinese children, pregnant and lactating women and infants were found during the recent revision of Chinese dietary reference intakes (DRIs) of iodine in 2013 by the applicant Pro. Zhang, and no systematical research about compensatory mechanism of iodine nutrition in lactating women was found yet. Previous study funded by United Nations International Children's Emergency Fund (UNICEF) cooperated with the international experts on iodine and key project of national natural science foundation of China under the guidance of Pro. Zhang, had obtained the safe intake range of iodine from children and pregnant women, more than 10 joint papers were published, which laid the foundation of population study and international cooperation.

Studies indicate lactating women and their infants are the most vulnerable population to the adverse effects of iodine deficiency and excess, and because of the complexity of the study of lactating mothers and infants and the difficulty of sample collection, it is proposed to overcome this problem with the help of the advanced dried blood spot technique and iodine balance study method from the panel of Iodine Global Network (IGN). Investigators intend to recruit 900 late pregnant women in Tianjin, Hebei and Shandong;then establishing a dynamic cohort of breastfeeding mother-infant to 6 months after delivery to investigate the physiological compensatory mechanism of iodine nutrition in lactating women with the combination of animal experiment; On the other hand, 180 exclusively breastfeeding mother and infants within postpartum 0-6 months, will be selected to participate the iodine balance study.

Optimized method of iodine balance experiment of infants by both foreign partner and Pro. Zhang's team will be adopted to obtain the first iodine balance value of Chinese lactating women and infants,providing the direct evidence for the revision of DRIs in lactating women and infants, and guide the lactating women proper iodine supplementation to meet the requirements of both mothers and infants.

Inclusion Criteria:

• Women in their third trimester:⑴Aged 18-40, who have lived in the area for 1 year;⑵Single pregnancy;⑶Those who have no special dietary habits;
• Lactating woman:⑴Breastfeeding a child;⑵Normal thyroid function during pregnancy;⑶Term delivery (38-42 weeks of gestation);
• Infants and young children:⑴0-6 months;⑵Breastfeeding;⑶Full term (38-42 weeks of gestation);⑷Normal birth weight (2500-4000g);⑸The thyroid-stimulating hormone (TSH) value of heel blood was normal;⑹Apgar score 8.

Exclusion Criteria:

• Women in their third trimester:⑴Smoking or drinking;⑵Iodized drugs or contrast agents have been used in the past year;⑶A history of thyroid disease, autoimmune disease, endocrine disease, heart disease, chronic disease or family hereditary disease, etc;
• Lactating woman:⑴A history of thyroid disease, autoimmune disease, endocrine disease, heart disease, chronic disease or family hereditary disease, etc;
• Infants and young children:⑴Have pathological jaundice, autoimmune disease, congenital heart disease and other diseases.

Women in the third trimester (18-40 years old), lactating women (38-42 weeks of gestation) and infants (0-6 months), all from Tianjin, Hebei and Shandong