The median UIC of children in this study was 68 μg/L, which falls between 50–99 μg/L, and according to WHO/UNICEF/ICCIDD, indicates that New Zealand children were mildly iodine deficient. Mild iodine deficiency has also been reported in Australian schoolchildren with a median UIC ranging from 74‒143 μg/L , and more recently in 14‒15 yr girls from the United Kingdom with a median UIC of 80 μg/L . In a smaller study in New Zealand on a random sample of 300 schoolchildren 8‒10 yr, a similar median UIC of 66 μg/L was found . In this earlier study, blood samples were not obtained from children but thyroid volume was measured by ultrasonography. Using the 1997 cut‒offs for thyroid volume, which were the only available reference data at that time , 10% of the children in this smaller study had goiter; when the data were reanalyzed using revised cut‒offs for thyroid volume , approximately 30% of the children were classified as having goiter . Although thyroid volume was not measured in the national survey reported here, the similar method of recruitment and median UIC between the earlier study of 8‒10 yr olds and this survey suggests that up to a third of New Zealand children would have had goiter in 2002.
The median Tg concentration of children in this survey of 12.9 μg/L falls within the range of 10.0‒19.9 μg/L, and further supports the results of the urinary analysis that New Zealand children were mildly iodine deficient. In healthy individuals free of thyroid disease, Tg increases with an increase in thyroid volume, thus Tg is an indirect index of goiter. The elevated Tg found in this survey substantiates our view that a large proportion of New Zealand children would have had goiter when the survey was conducted. We found a significant inverse relationship between age and Tg concentration. Djemli et al.  also found that Tg concentration decreased with age in children, in contrast to Zimmermann et al. who reported that the difference in Tg with respect to age was small . Of particular interest in this study was the finding that New Zealand children with an UIC ≥100 μg/L had a Tg concentration of 10.3 μg/L, providing evidence that the 10 μg/L cut-off proposed by WHO/UNICEF/ICCIDD for classifying mild iodine deficiency is appropriate for population studies of schoolchildren.
Because of the large intra‒ and inter‒individual variation in UIC , a limitation of UIC is that it can only be used to assess the iodine status of a population, but not of the individuals in that population. There is considerable interest, worldwide, in the development of an index of iodine status that can be used to assess iodine deficiency and its severity in individuals. Indeed, compared to other micronutrients such as iron and many vitamins, there is currently no biochemical measure to diagnose mild iodine deficiency in an individual. Soldin et al. used data from NHANES III and found that there was no relationship between UIC and TSH or T4 concentration . Serum Tg concentration holds promise as an index of individual iodine status, but further studies are needed to determine its’ specificity and sensitivity with regard to iodine deficiency. Furthermore, Tg concentration can be affected by a number of factors including interassay variability and the presence of Tg antibodies (Tg‒Ab), which can elevate the concentration of Tg. Zimmermann et al.  developed a dried blood spot method for Tg determination, however, to our knowledge, this method is not widely used nor been reproduced in other countries. Regardless of whether Tg can be used to identify iodine deficiency in an individual, our study shows that the median Tg concentration of a group of children can be used as an index of iodine status for a population, and if the dried blood spot method was used, could eliminate the need to collect urine samples under difficult field conditions.
The (geometric) mean TSH concentration of New Zealand schoolchildren of 1.7 mU/L was similar to values published in two other studies of children [23, 26] but lower than that reported in a 6–14 year old Austrian hospital‒based pediatric population . In contrast, higher median TSH concentrations were observed in American children 12–19 yr from NHANES III (i.e. girls: 1.3 mU/L and boys: 1.5 mU/L) . Most studies have found that TSH decreased with age, however, we did not observe an age effect in New Zealand schoolchildren. Furthermore, we did not find TSH concentration differed by sex, although both age and sex specific reference ranges for TSH do exist.
The mean fT4 concentration of children in this survey was 14.9 pmol/L and there was a significant effect of both age and ethnicity on fT4. A decrease in fT4 with increasing age has been observed in most other studies of children [23, 26, 29]. As with TSH, there are small differences in the actual mean fT4 concentration for children obtained from different studies, including our New Zealand data, however, this likely reflects methodological differences with the types of assays used to determine fT4. To our knowledge, this is the first study to find an effect of ethnicity on fT4 concentration, such that Caucasian children (i.e. NZEO) had a lower fT4 than Māori or Pacific children. The mean fT3 concentration of children in this survey was similar to other studies [27, 29]. We found no effect of age or sex on fT3 concentration, in agreement with the findings of Soldin et al. , however, Kapelari et al.  did report a small decline in fT3 concentration with increasing age. In this survey, New Zealand schoolchildren with a low UIC (i.e. either <50 μg/L or <100 μg/L) had significantly higher fT3 concentration than children with a UIC above these cut-offs, however, this difference was only 0.1 pmol/L and is unlikely to have biological significance. The mean fT3, fT4, and TSH concentrations for these children fell within normal reference ranges, which was not unexpected as changes in these indices only occur in moderate to severe iodine ficiency. In studies whose aim is to assess iodine status in schoolchildren, particularly in developed countries where iodine deficiency is unlikely to be severe, only measures of UIC and Tg are required.
This survey is one of the largest recent studies of the iodine status of schoolchildren published to date, but does have limitations. The serum samples had undergone up to four freeze‒thaw cycles prior to analysis, although we found that freeze‒thaw cycles had no effect on either TSH or Tg concentration. We used plasma samples for the determination of fT3 and fT4, which had a recovery >97% for both fT3 and fT4 compared to serum samples from the same individuals. We did not measure Tg‒Ab in our study due to limited financial resources. Zimmermann et al. suggests that screening for Tg‒Ab may not be necessary as a number of studies report a low prevalence (i.e. <5%) of Tg‒Ab in children .