Does the use of mean or median Z-score of the thyroid volume indices provide a more precise description of the iodine deficiency disorder status of a population?

Institute for Medical Research, Kuala Lumpur, Malaysia. Icfoo@imr.gov.my

Endemic iodine deficiency is largely an environmental problem affecting whole populations. Currently, thyroid volume data from a population are analyzed with the sole objective of obtaining an estimate of goitre prevalence using +97th percentile or +2 standard deviations of an appropriate reference as cut-off. This paper proposes an alternative approach to the analysis and presentation of thyroid volume data using Z-scores (standard deviation scores) of the thyroid volume indices such as thyroid volume-for-age or thyroid volume-for-body surface area. The calculation of the summary statistics of the Z-scores, such as mean or median, provides an alternative to the prevalence-based approach for expressing severity of iodine deficiency disorders (IDD). An advantage of the mean or median Z-score is that it describes the thyroid volume profile (and therefore the IDD status) of the entire population directly, unlike goitre prevalence which gives information only about the extremes of distribution. The frequency curve or histogram of the Z-scores provides a complete picture of the whole distribution. Although qualitatively similar conclusions on IDD severity can be drawn from both analytical approaches, only the Z-score system is able to capture adequately the trends or changes in thyroid size over time, and to establish whether a previously iodine-deficient community's thyroid volume profile has returned to 'normal' (as indicated by a distribution that is not significantly different from that of the reference) following intervention. As a continuous variable, Z-scores are particularly useful for the analysis of data from populations where the sample size is relatively small, or where many individuals lie outside the extreme percentiles of the reference population. In view of its advantages in the context of activities based on single and multiple measurements, the Z-score system is to be preferred for the reporting and use of thyroid volume indices. A desirable consequence of this preference is that national goals will be oriented towards an improvement of the overall thyroid volume profile of the population, rather than just a reduction of the number of individuals at the extremes.