Quantification of urinary iodine: a need for revised thresholds

Objective: To compare different possibilities of reporting the iodine supply in the same urine samples. Indeed, in field studies, urinary iodine concentration (I/L: mug I/L, mumol I/L, I/creatinine: mug I/g creatanine, mumol I/mol creatinine) is more readily available than excretion (I/24h mug I/24 h, mumol I/24h). However, confusion exists regarding the comparability of iodine supply based upon I/L, I/creatinine and I/24h, which for decades have been regarded as biochemically equivalent. Design: We compared I/24h, I/L and I/creatinine in accurate 24 h collections of urine and I/L and I/creatinine in 47 spot urine samples. Patients: A total of 13 subjects (Bern n = 7, Brussels n = 6) collected a total of 110 precise 24 h urine collections (Bern n = 63, Brussels n = 47). The subjects from Brussels also took a spot sample at the beginning of each 24 h collection. Results: Iodine supply in both places was mildly deficient according to the criteria of WHO; all but one collection indicated an intake of >0.39 mumol I/24h (>50 mug I/24h). The same data presented as I/creatinine (or I/L) indicated an iodine intake of <0.39 (<50 mug I/24h) in 5% (24%) of the samples in Bern and 23% (57%) in Brussels. Similar findings were observed for 47 spot samples. Whatever the cut-off selected, I/creatinine and I/L were systematically lower than I/24h (P < 0.0002). Creatinine showed smaller CV than volume but did not perform better in defining iodine intake. Conclusion: Considering I/24h as a reference, both I/creatinine and I/L clearly underestimate the iodine intake in subjects with adequate proteoenergetic intake. The significant deviations observed illustrate the urgent need for establishing separate ranges for I/24h, I/creatinine and I/L. In population studies, these deviations might even be larger.