Objectives: To quantitatively assess the intra- and inter-individual variation of urinary iodine concentration in Japanese to determine whether urinary analysis is applicable to assessing habitual iodine intake in subjects on an individual basis.
Methods: Five urine samples (first void) were taken from each of the 14 healthy female subjects at 2-3 week intervals over 4-5 months. Information on diet and medication use on the previous day of urine sampling was obtained by a questionnaire during each urine sampling. The concentration of iodine in urine samples was measured by inductively coupled plasma mass spectrometry (ICP-MS). Intra- and inter-individual variation was assessed by intra class correlation coefficient (ICC).
Results: The median concentration of iodine in 70 urine samples was 91 µg/g-cre (range: 15-4400 µg/g-cre). The mean iodine concentration in urine samples from subjects who took iodine-rich foods/medications on the day before sampling was statistically significantly higher than that from subjects who did not take such foods/medications (p < 0.01, t-test). The ICC of urinary iodine concentration of the 14 subjects was 0.55, indicating good reproducibility; however, this was 0.28 when one subject who routinely used an iodine-containing gargle was excluded from analysis.
Conclusions: Urine sampled on a single occasion is not a suitable medium for the assessment of long-term intake levels of iodine in subjects on an individual basis.