Background: Median urine iodine concentration (UIC; lg/L) in spot urine samples is recommended for monitoring population iodine status. Other common measures are iodine:creatinine ratio (I/Cr; lg/g) and estimated 24-hour urine iodine excretion (UIE; I/Cr · predicted 24-hour Cr; lg/day). Despite different units, these measures are often used interchangeably, and it is unclear how they compare with the reference standard 24-hour UIE. Methods: Volunteers aged 18–39 years collected all their urine samples for 24 hours (n = 400). Voids from morning, afternoon, evening, overnight, and a composite 24-hour sample were analyzed for iodine. We calculated median observed 24-hour UIE and 24-hour UIC, and spot UIC, I/Cr, and two measures of estimated UIE calculated using predicted 24-hour Cr from published estimates by Kesteloot and Joosens (varies by age and sex) and published equations by Mage et al. (varies by age, sex, race, and anthropometric measures). We examined mean differences and relative difference across iodine excretion levels using Bland–Altman plots. Results: Median 24-hour UIE was 173.6 lg/day and 24-hour UIC was 144.8 lg/L. From timed-spot urine samples, estimates were: UIC 147.3–156.2 lg/L; I/Cr 103.6–114.3 lg/g, estimated 24-hour UIE (Kesteloot and Joosens) 145.7–163.3 lg/day; and estimated 24-hour UIE (Mage) 176.5–187.7 lg/day. Iodine measures did not vary consistently by timing of spot urine collection. Compared with observed 24-hour UIE, on average, estimated (Mage) 24-hour UIE was not significantly different, while estimated 24-hour UIE (Kesteloot and Joosens) was significantly different for some ethnicity/sex groups. Compared with 24-hour UIC, on average, spot UIC did not differ. Conclusions: Estimates of UIC, I/Cr, and estimated 24-hour UIE (I/Cr · predicted 24-hour Cr) from spot urine samples should not be used interchangeably. Estimated 24-hourUIE, where predicted 24-hour Cr varies by age, sex, ethnicity, and anthropometric measures and was calculated with prediction equations using data from the sample, was more comparable to observed 24-hour UIE than when predicted 24-hour Cr was from published estimates from a different population. However, currently no cutoffs exist to interpret population estimated 24-hour UIE values.
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