Universal salt iodization (USI) is the recommended strategy for controlling iodine deficiency[18]. The iodization level has been changed three times in order to fine-tune the National IDD Elimination Programme since it was initialized in 1994[19].Wuhan has chosen 25 mg/kg(18–33 mg/kg) in the last change of iodization level in 2012 in order to avoid not only iodine deficiency but also iodine excess taking into account the actual iodine nutritional status of local population. In the present study, Wuhan has a high level of iodized salt coverage rate, which has guaranteed adequate iodine nutrition of population in Wuhan. Significant difference was not observed between UIC and different iodine content of table salt, in spite of the lowest UIC was in the inadequately iodized salt group.
The MUI value was the most commonly assessed indicator of iodine nutrition for the sampled population. The MUI of the present study was 247.50 ug/L, fell in the level of more than adequate recommended by international organizations [13], which had met the national iodine deficiency elimination standard. Compared with iodine nutrition of other parts of China, the iodine nutrition of Wuhan which located in central of China was relatively adequate. The MUI of Zhejiang Province which located in the eastern coastal area of China was 178ug/L[20].Similar to the results of this study, the MUI of Chongqing, which located in western China was found to be 222.00ug/L[17].The present result indicated that the situation of iodine deficiency in Wuhan have been corrected, and the comprehensive prevention and control measures based on salt iodization for 26 years have achieved remarkable achievements. However, excessive iodine nutrition (≥ 300 µg/L) was observed among 36.70% of the children, especially in Hannan district ,and 14.00% of the children had UIC below requirement(< 100 µg/L). The reason on the one hand may be related to the increased consumption of processed foods for children. At present, various foods with high salt content were produced on the market for children’s tastes, such as potato chips, seaweed, instant noodles, mustard, bacon and so on. Children may consume more iodine when they eat these high-salt foods regularly, which can cause excessive iodine intake. Thus they should be instructed appropriately to adjust the intake of iodine-containing foods and carried out long-term dynamic monitoring to ensure their proper iodine nutritional status since excess iodine nutrition could result in iodine-induced thyroid disease, such as hyperthyroidism and autoimmune [21]. On the other hand, the ability of detection of iodine deficiency laboratory varied in different district, which may also lead to too high or too low UIC detection value of children. There were statistically significant differences in UIC among children of different genders, ages and administrative districts, but not with height, BSA and Tvol .
UIC assesses iodine nutrition only at the time of measurement, whereas thyroid size reflects long-term iodine nutrition of an individual or a population. Moreover, goiter prevalence responds slowly to changes in iodine intake as thyroid size may not return to normal for months or years [5].In our present work, Tvol determinations were made by a single experienced operator which free of inter- observer variation, a factor known to be one of the reasons resulting in the current disagreements surrounding normal Tvol values for children[22] .The median Tvol of Wuhan in boys and girls were 2.57 ml and 2.56 ml respectively, smaller than that of African boys and girls, which found to be 3.32 ml and 3.53 ml which may due to genetic or environmental factors, and smaller than that of 2.9 ml in Chongqing and 3.1 ml in Zhejiang Province observed in 2013[23].In 3–14 year-old Spanish children, significant sex differences in Tvol were found for 8–12 years of age[1].However, no such gender differences were found in our study and in most iodine-sufficient areas’ studies[24].In the Netherlands, Wiersinga et al observed that, it was not until the age of 14 that the gender difference in thyroid volume began to be greater than the BSA difference [25].
In our present finding, although positive correlation was observed between thyroid volume and either age or body surface consistent with other study[26],a multiple linear regression showed that only BSA significantly predicted thyroid volume. However, in the Ashanti Region of Ghana, Daniel Gyamfi showed that only age significantly predicted thyroid volume which was conducted in 6–12 years old children, although which factor had the greatest impact on thyroid volume remained to be studied[27]. It may be that the years of 8–10 cannot be enough to reflect the difference in Tvol, and the result may be different after expanding the age range in our study.
There were many factors that affected thyroid volume, among which the relationship between UIC and thyroid volume was controversial. Some previous researches indicated that the relationship between them varied significantly in different regions. High goiter rate in children with high median UIC was observed in China with high iodine content in drinking water[28]. In China's Hebei, Shandong and other areas where iodine deficiency, iodine adequate, and iodine excess coexist, urinary iodine and thyroid volume have a "U"-shaped curve relationship, that is, both insufficient and excess iodine nutrition can cause glands to become enlarged[29]. Multivariate linear regression in the present study found that BSA could explain only 20.7% of the thyroid volume, which meant that 79.3% of thyroid volume was determined by other factors, such as race, geographical area and diet habits[16].Some researches indicated that iodine nutrition was the most important factor affecting thyroid volume in iodine-deficient areas, while dietary habits and genetic differences in growth and development played the most important role in thyroid volume in an iodine-sufficient area[25].And some studies showed that the differences in thyroid volume were related to genetic and environmental factors[30].In our study, the thyroid volume was not in significant correlation with UIC, indicating that iodine nutritional status had little effect on Tvol of school-aged children in Wuhan, consistent with the result of Zhejiang Province[23].
In the present study, consistent with several other studies[27], we presented the data on BSA and age-specific thyroid volumes. The P97 thyroid volumes of both boy and girl of school-aged children observed in this study were generally higher than that reported by WHO/ICCIDD in 2007 irrespective of whether it was expressed as a function of age and BSA. The Chinese criteria employed in this study only considered the factor of age without adjusting for BSA and gender, which may be high for iodine deficiency areas like Wuhan, and ignored gender differences and children’s variant body development of the same age [17]. Thus the goiter prevalence using the criteria may be underestimated. But whether the standards of WHO were appropriate to assess the goiter of children in Wuhan remained to be further studied. Formal international recommendations advised that standard references of thyroid volumes should be established based on a nation or a region [1].Therefore, larger studies were needed to gather more data to explore the population-specific reference range such as that done in Sweden on BSA-specific reference by age[31].
There were some limitations in this study. Firstly, we didn’t collect the thyroid function and anti-thyroid antibody of children, which may be useful for thyroiditis diagnose and explain some of the identified goiter [32].Secondly, some BSA groups have insufficient sample size, such as 0.7 m2 of boys and 1.6 m2 of girls, so the values of P97 for such BSA could not be presented in the Fig. 2 and some outliers made the Tvol not increase steadily with the increase of BSA.