TABLE 20.

Justification and considerations for the use of UIC in application of the EAR method¹

• The population distribution of UICs is typically skewed toward lower intakes with a scattered tail of high intakes due not only to variations in iodine intake between individuals but also to high day-to-day variability of iodine intakes for individuals in the target population

• The sample size of the group takes interindividual variation into account (when properly powered) but does not account for individual day-to-day variations (147, 148)

• Therefore, before UIC data from a target group can be used for this application, the UIC distribution in populations with large variations in iodine intakes must be adjusted for intraindividual variation (144, 146)

• This can be done by collecting ≥2 repeated samples from the same individual in a subset of the study population

• The information on variability obtained can then be used to correct for within-person variation in the group (149)

• This approach adjusts the observed intake distribution closer to the mean (i.e., to more closely resemble a normal intake distribution of habitual intakes) (146, 150)

Advantages and additional considerations:

• The normalization of the distribution generates a prevalence estimate that is closer to the true prevalence of inadequacy

• Adjustment will generally reduce the prevalence compared with noncorrected distributions, except in populations with low or homogeneous iodine intakes (140)

• In populations with low iodine intake (i.e., mean intakes below the EAR), the distribution is typically skewed to lower intakes and adjustment may underestimate the prevalence

• In countries with a well-functioning salt iodization program, the iodine intake is less variable and the population distribution is close to normal without correction, as observed in Swiss schoolchildren (151)

• A further advantage of collecting a repeat urine sample and accounting for intraindividual variation is that the required sample size can likely be smaller than the 500 samples now recommended for population assessment (147, 148)

¹EAR, Estimated Average Requirement; UIC, urinary iodine concentration.