Iodine Status Assessment in South African Adults According to Spot Urinary Iodine Concentrations, Prediction Equations, and Measured 24-h Iodine Excretion

Abstract

The iodine status of populations is conventionally assessed using spot urinary samples to obtain a median urinary iodine concentration (UIC) value, which is assessed against standard reference cut-offs. The assumption that spot UIC reflects daily iodine intake may be flawed because of high day-to-day variability and variable urinary volume outputs. This study aimed to compare iodine status in a sample of South African adults when determined by different approaches using a spot urine sample (median UIC (MUIC), predicted 24 h urinary iodine excretion (PrUIE) using different prediction equations) against measured 24 h urinary iodine excretion (mUIE). Both 24 h and spot urine samples were collected in a subsample of participants (n = 457; median age 55 year; range 18–90 year) in the World Health Organization Study on global AGEing and adult health (SAGE) Wave 2 in South Africa, in 2015. Kawasaki, Tanaka, and Mage equations were applied to assess PrUIE from predicted urinary creatinine (PrCr) and spot UIC values. Adequacy of iodine intake was assessed by comparing PrUIE and mUIE to the Estimated Average Requirement of 95 µg/day, while the MUIC cut-off was <100 µg/L. Bland Altman plots assessed the level of agreement between measured and predicted UIE. Median UIC (130 µg/L) indicated iodine sufficiency. The prediction equations had unacceptable bias for PrUIE compared to measured UIE. In a sample of adult South Africans, the use of spot UIC, presented as a group median value (MUIC) provided similar estimates of inadequate iodine status, overall, when compared to EAR assessed using measured 24 h iodine excretion (mUIE). Continued use of MUIC as a biomarker to assess the adequacy of population iodine intake appears warranted.

Spot urine iodine levels below the WHO recommendation are not related to impaired thyroid function in healthy children and adolescents

“…To overcome this issue, some authors suggest measuring SUIC after multiple sampling or in 24 h-pooled samples [ 34 , 35 ]. Furthermore, in terms of SUIC values, repeated 24 h-sampling over a number of months is more reproducible, whereas a repeated collection of spot urine produces greater variation [ 36 ]. The most important advantage of spot urine collection vs. 24 h urine sampling is its practicability.…”

“…In large studies, in particular, it is practically impossible to collect 24 h-pooled samples and, in any case, as the number of participants involved in the study increases, the day-to-day and the intra-day variation of SUI levels tends to even out [ 2 ]. Moreover, the classification of a cohort as iodine sufficient or insufficient by the measurement of SUIC has been shown to exhibit excellent reproducibility over one month [ 36 , 37 ]. As such, SUIC measurement is not appropriate for indicating iodine deficiency in a single individual, but it is very useful for comparing different cohorts or monitoring long-term changes.…”

Urinary Iodine Concentration in a Cohort of Adult Outpatients with Thyroid Diseases in Liguria 14 Years after the Law on Salt Iodization

“…Indeed, this modality not only provides values for the assessment of iodine intake in school-age children, but is also applicable to adults, except pregnant and lactating women [19]. However, UIC determined on single random urine sample is inevitably affected by daily variations and variable urinary volume, and for this reason its accuracy is still debated [25,26].…”

Magnitude of iodine deficiency disorder and associated factors in Dawro zone, Southwest Ethiopia; the hidden hunger: a cross-sectional study

“…The study focused on school age children (SAC) aged 6-12 years. According to WHO/ UNICEF/IGN, these groups are a useful target for IDD surveillance because of their combined high vulnerability, easy access, applicability to a variety of surveillance activities and relative representativeness of the study population [2,30].…”