Based on the investigators'previous study on upper safe level of iodine on children from areas with high iodine in drinking water, this study will conduct an epidemiological study on 600 children living in areas with high water iodine content but the actual iodine content in participants' drinking water is not so high, while no supply of iodized salt.

Meanwhile, in order to get iodine dietary recommended nutrient intake (RNI) values of children. The investigators will conduct a iodine balance experiment on children with sufficient iodine nutrition. We will randomly select thirty healthy students for a three-day iodine balance experiment from the cross-sectional study. The investigators will use the regression model to obtain an iodine intake value when children at"zero iodine"balance. Combine the study at population and individual level, the investigators will obtain data on dietary recommended nutrient intake (RNI) of iodine for Chinese children, to properly guide children iodine supplementation to avoid iodine deficiency, which also will provide data for the update of Chinese dietary reference intakes of iodine.