Urinary Iodine Concentration
Urinary iodine concentration
Urinary iodine concentration is the recommended biomarker of population iodine status (16, 17).
Excretion of iodine in the urine reflects recent iodine intake and UIC is the primary biomarker for population iodine nutrition (9, 16, 23). UIC reflects the risk of inadequate/excessive iodine intakes in a population, and in turn, the risk of developing thyroid disorders (9).
In healthy, iodine-replete adults >90% of dietary iodine is absorbed from the small intestine and >90% is excreted within 24-48 hours (23). UIC is conventionally measured in spot urine samples and expressed as a population median in μg/L (9, 16). WHO thresholds for the median UIC reflect insufficient, adequate and excessive iodine intakes. Assessment of individual iodine status using spot UIC is not recommended due to intra-diurnal variations in intake (24-26). At population level, variations are considered to even out with adequate sample size (16, 23), and correction for hydration (e.g. with creatinine) is typically not applied (16). (For more details on urine correction, see the page on Common Methods).
UIC surveys have mainly been collected in school-age children, since they are assumed to represent much of the population, except for pregnant and lactating women, and infants. However, assessment of iodine status in women of reproductive age, pregnant women and at risk groups is encouraged as resources allow (16, 17).
Note: assessment of the iodine status of lactating women using UIC alone may be inadequate since iodine is also present in breast milk; breast milk iodine concentration has been proposed as a more reliable biomarker of iodine status in lactating women (15). See Other Methods.
- For iodine analysis, all subject and laboratory consumables (plastic cups, syringes, infant nappies/diapers, collection bags, Eppendorf tubes, pipette tips etc.) should be certified trace-element free, or pre-tested for iodine contamination before use.
- Urine should be collected without using preservatives, and urine that has been previously used for dipstick analysis e.g. for glycosuria, should not be used due to risk of contamination.
- Pooled in-house urine quality controls need to be prepared. Ideally, persons (e.g. laboratory colleagues) with a low and high intake should provide a urine sample, which is aliquoted for use as internal quality control standards. After urine collection, the intended quality controls should be analysed at least 10 times across three different plates and compared with existing standards and controls. If the UIC obtained from volunteers is lower than that needed for adequate low and high quality controls for assay purposes, the urine can be spiked with iodine standard to obtain the desired UIC concentration. If needed, control samples can also be obtained from the CDC EQUIP scheme – see below.
- Laboratories conducing urinary iodine analysis by ICP-MS or the Sandell-Kolthoff method are strongly encouraged to participate in the EQUIP accreditation scheme run by CDC to ensure the quality of urinary iodine analyses. See Page 4 for more information.
Stability and storage:
Spot urine samples obtained in the clinic or at the study site should be transferred to the laboratory for processing. Transport does not have to be cold or frozen as the iodine in urine samples is stable. However, for the comfort of the laboratory operator, urine samples are best kept refrigerated.
Once samples have been aliquoted into the required number of aliquots, they should be frozen at -20°C or colder until analysis.
Iodine is stable in urine when frozen at -22°C or colder for 15 years (30). Iodine is stable in urine for repeated freeze-thaw cycles, however freeze-thaw cycles beyond requirement should be avoided to prevent unnecessary evaporation.