Iodine is an important micronutrient that is required for the production of thyroid hormone, which is crucial for neurodevelopment during early life and various metabolic processes at all life stages. Dietary iodine sources include iodized salt and foods that have been fortified with iodine, including some dairy products and grains (1). Due to substantial day-to-day variation of iodine intake, iodine status is not able to be well characterized in individuals. Instead, measures of populationlevel iodine status have been established to determine whether large groups and/or geographical regions are iodine sufficient (2). Accepted measures of population-level iodine status by WHO are 1) the frequency of elevated serum thyroid-stimulating hormone (TSH) concentrations in newborns and in school-age children, 2) median urinary iodine concentrations (MUICs; which can also be used in pregnant women), 3) the frequency of elevated serum thyroglobulin concentrations, and 4) the prevalence of goiter (3). The choice of which measure to use in determining the nutritional adequacy of iodine across large population groups depends in part on the availability of local personnel, resources, convenience, and cost.

In the current issue, Wassie and colleagues (4) report the findings of a systematic review of several of these population iodine status measures, as ascertained from the available published literature to October 2018. There were 57 included studies among 30 publications, from which the relative agreements between MUICs, frequency of TSH elevation, and frequency of goiter, as the selected markers of population iodine nutrition, were assessed. Nearly 60% of the studies were conducted in countries classified as iodine-deficient, according to the overall classification of the country by either WHO or the Iodine Global Network (5, 6). Two measures were used as reference measures—MUICs and the prevalence of goiter in school-age children—from which their correlations with newborn TSH concentrations were determined. Overall findings showed that there was fair agreement between newborn TSH concentrations and the prevalence of goiter in school-age children, whereas there was a weaker correlation between newborn TSH concentrations and MUIC in school-age children. In addition, newborn TSH concentrations that were not obtained from cord blood or ideally within the initial 3–4 d following birth, as recommended by WHO (3), were found to be unreliable in the classification of population iodine status.