Highlight statistical problems with assessing population iodine status and suggest potential improvements.
Because urinary iodine concentration (UIC) is highly variable within an individual, a single value cannot define an individual’s iodine status. This intra-individual variability broadens the distribution of UIC values which can then over- or under-estimate the proportion of survey subjects falling below some cut-off, potentially resulting in failure to recognize a successful salt iodization program. Moreover, because the distribution of UIC values is typically non-normally distributed, the median is the appropriate measure of central tendency, and precision is rarely calculated. Even if it were, non-parametric statistical tests cannot easily account for loss of precision due to complex sampling. As a result, most surveys do not assess the statistical significance of a difference between the sample median and the cut-off point. One method can remove intra-individual variability so that the distribution of UIC values can be compared to the average iodine requirement. However, it requires duplicate urine specimens from a subsample of survey subjects, which is often difficult in surveys. In addition, because it needs special software to calculate, is not widely used.
Although UIC is currently the main indicator of population iodine status, it has limitations that should be more widely publicized to prevent spurious calculations of iodine deficiency prevalence. Methods to remove intra-individual variability could be made more user-friendly. Finally, other indicators of iodine status in individuals and populations, such as serum thyroglobulin, should be more thoroughly investigated.