Iodine deficiency and iodine excess in Jiangsu Province, China

Keywords:iodine deficiency, iodine excess, endemic goiter, drinking water, iodine intake, thyroid function, thyroid size, iodized salt, iodized oil, IQ, physical development, hearing capacity, epidemiology, meta-analysis, IDD, randomized trial, intervention, USA, Bangladesh, ChinaEndemic goiter can be caused both by iodine deficiency and iodine excess. Iodine deficiency was a public health problem in Jiangsu Province, China and has been eliminated through salt iodization in a majority of counties in Jiangsu. In Feng, Pei and Tongshan Counties in Xuzhou Municipality, endemic goiter has been found to be associated with iodine excess. This thesis describes the relationship of iodine intakes ranging from normal to excessive with thyroid size, thyroid function, goiter prevalence, intellectual development, physical development and hearing capacity, and the efficacy and effectiveness of iodine intervention programs in schoolchildren. An ecological relationship of iodine intakes ranging from normal to excessive with an enlarged thyroid size, an increased prevalence of goiter and a perturbed thyroid function has been found in three counties in Xuzhou Municipality. A negative relationship of iodine intake from normal to excessive with intellectual quotient (IQ) in schoolchildren aged >11 y indicates that excessive iodine intake is associated with a decline in IQ. This was supported by the results of a meta-analysis of 16 studies where iodine excess was found to be related to a small but significant deficit in intellectual attainment of schoolchildren. Reduction of iodine intake from excessive to normal is accompanied by a reduction in thyroid size, a decreased urinary iodine concentration and a partial normalization of thyroid function. It is now estimated that iodine excess is a public health risk for nearly 16 million people in 92 counties of 10 provinces in China. Comparison of thyroid volumes collected from schoolchildren in the US and in Bangladesh with those in Europe found that the new WHO reference for the upper limit of normal thyroid volume is not applicable worldwide. A randomized trial in an iodine deficient area showed that it takes 12 months for enlarged thyroid glands to reduce to normal if iodine supply is consistently adequate. This period is longer if the iodine supply fluctuates. An evaluation of the effectiveness of iodine intervention programs in 9 counties of Jiangsu Province indicated that iodine deficiency has been eliminated in these counties. Based on the evidence from the work described in this thesis, a maximum allowable iodine concentration can be set at 300μg/L in drinking water or at 800μg/L in urine of adults. Iodine concentrations above these levels should be regarded as a public health risk