ORIGINAL COMMUNICATION

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1 (2003) 57, & 2003 Nature Publishing Group All rights reserved /03 $ ORIGINAL COMMUNICATION : a need for revised thresholds C Als 1,2,3, C Minder 4, D Willems 5, HV Van Thi 6, H Gerber 2 and P Bourdoux 6 * 1 Institute of Pathology, Inselspital, University of Bern, Bern, Switzerland; 2 Institute of Clinical Chemistry, Inselspital, University of Bern, Bern, Switzerland; 3 Institute of Nuclear Medicine, Clinique Ste Thérèse, Luxembourg, Luxembourg; 4 Institute of Social and Preventive Medicine, Inselspital, University of Bern, Bern, Switzerland; 5 Laboratory of Clinical Chemistry-Centre Hospitalier Universitaire Brugmann, Brussels, Belgium; and 6 Laboratory of Pediatrics, Université Libre de Bruxelles, Brussels, Belgium Objective: To compare different possibilities of reporting the iodine supply in the same urine samples. Indeed, in field studies, urinary iodine concentration (I/L: mg I/L, mmol I/L, I/creatinine: mg I/g creatanine, mmol I/mol creatinine) is more readily available than excretion (I/24h mg I/24 h, mmol 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 ¼ 7, Brussels n ¼ 6) collected a total of 110 precise 24 h urine collections (Bern n ¼ 63, Brussels n ¼ 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 mmol I/24h (450 mg I/24h). The same data presented as I/creatinine (or I/L) indicated an iodine intake of o0.39 (o50 mg 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 (Po0.0002). 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. (2003) 57, doi: /sj.ejcn Keywords: Iodine deficiency; IDD; gold standard; urine *Correspondence: P Bourdoux, ULB-Laboratoire de Pédiatrie, Hôpital Universitaire des Enfants, Avenue J. J. Crocq 15, B-1020 Brussels, Belgium. pbourdou@ulb.ac.be Guarantors: PP Bourdoux, Dr C Als. Contributors: CA was involved in conception and design of the study, acquisition and collection of data, analysis and interpretation of data, and in writing and revising the article; CM performed analysis and interpretation of data, writing the article, and revising the article; DW was involved in acquisition and collection of data, interpretation of data; HVVT in the acquisition and collection of data, analysis of data, interpretation of data, writing and revising the article; HG in the conception and design of the study, interpretation of data, revising the article; and PB was involved in conception and design of the study, acquisition and collection of data, analysis and interpretation of data, writing and revising the article. Received 29 November 2002; revised 4 February 2003; accepted 6 February 2003 Introduction Iodine deficiency disorders (IDD) that affect more than 1 billion people worldwide are still a major concern in many countries. The World Health Organization (WHO) had decided to eradicate iodine deficiency (ID) by the year 2000 (WHO, 2001). Until recently, the prevalence of goiter and cretinism were the principal criteria for evaluating the severity of ID in endemic areas. However, the presence or absence of goiter is only a visible hallmark of inadequate iodine metabolism. Other more subtle clinical entities that are difficult to quantify, such as transient neonatal hypothyroidism or the rate of miscarriage, are common findings in ID (Bourdoux, 1993). Epidemiological investigations are then mainly based on biochemical analyses of urinary iodine, and desirably but infrequently on serum thyrotropin and thyroxine (Bourdoux, 1993). An accurate

2 1182 quantification of urinary iodine is then of the utmost importance for the implementation and follow-up of prophylaxis programs. Although the prevalence of goiter, at least in adults, does not change very rapidly, an additional, complementary measure of thyroid volumes by ultrasound may be interesting, at least in epidemiological studies. Iodine supply is the result of alimentary and pharmacological intakes of iodine. It is routinely evaluated via urinary excretion. In physiological conditions, losses by perspiration and feces are negligible (Follis et al, 1962; Vought et al, 1963) so that urinary iodine is regarded as a valuable index of iodine intake. Since its first description in 1937, the chemical analysis of iodide proposed by Sandell & Kolthoff (1937) was first applied to geological substrates such as rock, soil and water. Its application to human tissues and secretions became generalized about 40 y ago (Wolff & Chaikoff, 1948; Follis et al, 1962; Vought et al, 1964; Jolin & Escobar del Rey, 1965; Lauber, 1975; Bourdoux, 1988; Bürgi et al, 1990; Als et al, 1995; Haldimann et al, 1998) and several modes of presentation have been used for reporting data about iodine output. When precise 24 h urine collections could be obtained, iodine output was presented as an excretion in mg I/day or better mg I/24h. In field studies, quantification of iodine has been most often deduced from readily available spot samples and expressed as a concentration in mg I/g creatinine (mg I/g cr, mmol I/mmol cr) or, lately, in mg I/dL (Follis et al, 1962; Vought et al, 1963; Jolin & Escobar del Rey, 1965; Bourdoux et al, 1985; Bourdoux, 1988, 1998; Delange et al, 1997; Als et al, 2003). However, many authors still confusedly use the term excretion instead of concentration or try to simply extrapolate the former from the latter or report mean values for data that are not normally distributed. Moreover, switching from metric (mg) to molar (mmol) units has recently added to this confusion. As a result of these different presentations, comparison with ancient data and interpretation of new data is sometimes difficult or even impossible. The true impact of additional variables like volume/24h and creatinine/24h on the numerical value of urinary iodine is actually unknown. In the present study, our aim was therefore to compare, on the same samples, quantities which for years or decades have been regarded as biochemically equivalent: mmol I/24h (mg I/ 24h), mmol I/mmol cr (mg I/g cr) and mmol I/L (mg I/dL). Repeated 24 h urine collections were obtained from 13 healthy motivated volunteers in two European cities and spot samples were obtained from six of them in one of these two cities. The aim was to compare the quantities I/24h, I/cr and I/L in spot samples and 24 h collections in regard to popular but arbitrary thresholds of daily iodine supply. Subjects and methods Subjects All study subjects were healthy and had a sufficient proteoenergetic intake. They lived in two geographical regions with a temperate climate and different histories of iodine supply: Bern in Switzerland and Brussels in Belgium. In Bern, previous ID has been progressively corrected by a nationwide program of iodized salt (5, 10 and 20 mg KI/kg salt, introduced in 1922, 1962 and 1980, respectively). A State monopoly of salt production and trade coupled with legally fixed and identical prices for all sorts of salt has been essential for iodized salt to reach the largest part of the population (Bürgi et al, 1990). In Belgium, on the contrary, no centralized salt iodization and distribution program exists. Iodine supply has been heterogeneous; moderate-tointermediate ID has prevailed, and mild endemic goiter has been found in the eastern part of the country (Beckers et al, 1993). Members of the hospital, friends and relatives in Bern (n ¼ 7, ie, three males, four females, aged y, mean: 35 y, median: 30 y) and Brussels (n ¼ 6, ie, three males, three females, aged y, mean: 38.6 y, median: 37 y) collected a total of 110 (each: 7 12, mean: 9) precise 24 h urine collections between 1st November and 20th December In addition, the subjects from Brussels took a spot sample at the beginning of each 24 h collection. None of the healthy volunteers was taking iodine-containing medicine like amiodarone, thyroxine, triiodothyronine or vitamins and micronutrients; none was on a specific diet. The subjects continued their usual eating habits along the study period. Some subjects showed physiological peculiarities: three subjects from Bern drank fairly little, one subject from Brussels was a sportsman (male, aged 21 y), and another subject from Brussels had delivered 6 months before starting the sampling period and was still lactating. The ethical committee at the University of Bern has approved the study design (no. 77/96). Laboratory determinations After sufficient mixing of each 24 h collection, the volume was carefully measured and four aliquots were taken and immediately deep frozen for further determinations of iodine and creatinine. All analyses were performed in Brussels according to techniques previously described (Jaffé, 1886; Bourdoux, 1988). Calculations The final results were expressed as volume (L/24h), urinary excretion of creatinine (mmol cr/24h or g cr/24h) or iodine (mmol I/24h or mg I/24h) and as urinary concentrations of iodine (mmol I/mmol cr and mmol I/L, respectively, mg I/g cr and mg I/dL). The percentages of iodine results expressed as I/ 24h, I/cr and I/L found below several arbitrary thresholds of ID were compared. As no controlled metabolic studies on thresholds of ID exist, we chose for I/24h the somewhat arbitrary thresholds of 1.18 mmol or 150 mg (no ID), 0.98 mmol or 125 mg (marginal ID), 0.79 mmol or 100 mg (mild ID), 0.59 mmol or 75 mg (intermediate ID), 0.39 mmol or 50 mg

3 Table 1 Numerical data (L/24h, mmol cr/24h, mmol I/24h, mmol I/mmol cr, mmol I/L) from 24 h urine collections (n=63 in Bern, n=47 in Brussels) and from 47 spot urine samples in Brussels; mean7s.d., median and 95% CI, range, and CV 1183 Volume L/24h Creatinine mmol/24h I Excretion mmol/24h I/cr ratio mmol/mmol I Concent. mmol/l 24 h Collections All (n=110) Median % CI Range CV Significance* PvsI/24h F F F Po Po PvsI/cr F F F P= Bern (n=63) Median % CI Range CV Brussels (n=47) Median % CI Range CV Mean Brussels/mean Bern significance** P(Br vs Be) P= P= P= Po P= Spot samples Brussels (n=47) Mean7s.d. F F F Median F F F % CI F F F Range F F F CV Significance*** PvsI/24h F F F Po P= PvsI/cr F F F P=0.059 P= PvsI/L F F F P= P=0.041 *The value of P refers to comparison of excretion and concentration of urinary iodine. **The value of P refers to comparison of values in Bern and Brussels. ***The value of P refers to comparison of spot samples and 24 h collections. To convert mmol cr/24h into g cr/24h multiply by To convert mmol I/24h into mg I/24h multiply by 127. To convert mmol I/mmol cr into mg I/g cr multiply by To convert mmol I/L into mg I/dL multiply by (moderate ID) and 0.20 mmol or 25 mg iodine (severe ID). For comparison, we selected the following thresholds for I/cr: 0.13 (150), 0.11 (125), 0.09 (100), 0.07 (75), 0.04 (50) and 0.02 (25) mmol/mmol (mg/g) in molar and metric units, respectively and for I/L: 1.18 (15), 0.98 (12.5), 0.79 (10), 0.59 (7.5), 0.39 (5) and 0.20 (2.5) mmol/l (mg/dl). The ratios: concentration of iodine/excretion of iodine, that is, (I/cr/I/ 24h) and (I/L/I/24h) were calculated. The correlation between iodine, creatinine or volume with the anthropometric data (sex, age, body mass index and body surface area) is not shown in the present paper. Overall findings from Bern and Brussels were also compared. Statistical methods Data were expressed as mean7s.d. and/or median and 95%CI; range and CV were also calculated. The Mann Whitney test or the w 2 test was used to test the possible difference between groups. A statistical result was considered significant if Po0.05. Results Creatinine and volume (Table 1) Median volume for the h urine collections was 1.5 L/ 24h (mean: L/24h) with slight but significant

4 1184 difference between Bern and Brussels. Median creatinine excretion was 12.4 mmol/24h or 1.4 g/24h (mean mmol/24h or mean g/24h) with no significant difference between the two cities. Mean and median values of L/24h and cr/24h in each subject differed only little (data not shown). For the 110 collections, the CV of L/24h was twice as high as the one of cr/24h (55.2 vs 26.5%). As indicated in Table 1, volume and creatinine excretion were found in the expected ranges for well-fed populations (Joossens et al, 1980). Some individuals however had extreme values: the sportsman from Brussels had the highest creatinine values, ranging from 17.2 to 22.1 mmol cr/24h or from 1.95 to 2.50 g cr/24h (mean: 20.0 mmol cr/24h or mean: 2.26 g cr/24h). A small-sized female subject from Brussels had low creatinine values (range: mmol cr/ 24h or g cr/24h, mean: 8.9 mmol/24h or 1 g cr/ 24h). Median volumes in three subjects from Bern, who drank little, were rather low: L, L and L. Iodine (Table 1) Compared to values expressed as I/24h (taken as reference values), iodine values calculated as I/cr or I/L were significantly decreased down to 69% of the reference value. A separate analysis of data from Bern and Brussels showed similar decreases but with different amplitudes (71% in Bern and 58% in Brussels). Iodine concentrations (I/L and I/cr) in spots were fairly comparable to I/L or I/cr obtained from 24 h collections. However, I/L and I/cr in spots showed significant deviations if compared to I/24h. The comparison of data from Bern and Brussels showed that the mean daily excretion of iodine was slightly but significantly higher in Bern than in Brussels (0.91 compared to 0.82 mmol I/24h or 115 compared to 104 mg I/24h). Median iodine supply, expressed as I/24h, corresponded to marginal or mild ID in Bern and Brussels according to criteria endorsed by WHO at the beginning of the study (WHO, 1994) and at the time of publication (WHO, 2001). Table 2 Percentages of samples (24 h collections and spot samples) found below threshold levels of iodine (I/24h, I/cr or I/L) Percentages of samples below threshold levels of iodine Arbitrary classification of iodine Deficiency Corresponding threshold of urinary iodine All (n=110) Bern (n=63) Brussels (n=47) (w 2 )* P* No ID 1.18 mmol I/24h (150 mg I/24h) F F mmol I/mmol cr (a) o mmol I/L (b) NS:o0.20 Spot: 1.18 mmol I/L F F (c) o0.02 Marginal ID 0.98 mmol I/24h (125 mg I/24h) F F mmol I/mmol cr (a) o mmol I/L (b) o0.05 Spot: 0.98 mmol I/L F F (c) o0.01 Mild ID 0.79 mmol I/24h (100 mg I/24h) F F mmol I/mmol cr (a) o mmol I/L (b) o0.001 Spot: 0.79 mmol I/L F F (c) o0.001 Intermediate ID 0.59 mmol I/24h (75mg I/24h) F F mmol I/mmol cr (a) o mmol I/L (b) o0.001 Spot: 0.59 mmol I/L F F (c) o0.001 Moderate ID 0.39 mmol I/24h (50mg I/24h) F F mmol I/mmol cr (a) o mmol I/L (b) o0.001 Spot: 0.39 mmol I/L F F (c) o0.001 Severe ID 0.20 mmol I/24h (25mg I/24h) F F mmol I/mmol cr F F 0.20 mmol I/L (b) NS:o0.10 Spot: 0.20 mmol I/L F F (c) o0.02 *w 2 test for data from Bern and Brussels together (from Brussels only for SPOT samples): (a)i/cr vs I/24h, (b) I/L vs I/24h, (c) spot: I/L vs I/24h. To convert mmol I/24h into mg I/24h multiply by 127. To convert mmol I/mmol cr into mg I/g cr multiply by To convert mmol I/L into mg I/dL multiply by 12.7.

5 No of samples (%) Urinary Iodine μmol I/24h μmol I/mmol Cr μmol I/L μmol I/24h μmol I/mmol Cr μmol I/L Figure 1 Comparison of the percentages of samples found below the arbitrary thresholds used to define iodine deficiency or sufficiency. Kdenotes Po0.05, KKPo0.02 and KKKPo0.001 for the comparison of mmol I/24h with mmol I/mmol Cr or mmol I/L by the w 2 test. To convert mmol I/24h into mg I/24h multiply by 127. To convert mmol I/mmol cr into mg I/g cr multiply by To convert mmol I/L into mg I/dL multiply by Classification of iodine data in regard to thresholds of iodine supply (Table 2) Compared to I/24h, the percentages of samples found below the arbitrary thresholds (1.18 (15 mg/dl), 0.98 (12.5 mg/dl), 0.79 (10 mg/dl), 0.59 (7.5 mg/dl), 0.39 (5 mg/dl) and 0.20 mmol (2.5 mg/dl) or mmol/l) showed highly significant differences when expressed as I/cr or I/L. For instance, a single value (from Bern) indicating moderate ID (o0.39 mmol I/24h or o 50 mg I/ 24h) was found in the 110 samples (Tables 1 and 2 and Figure 1). However, if the threshold was considered to be o0.045 mmol I/ mmol cr (or o50 mg I/gcr)oro0.39 mmol I/L (or o5 mg I/dL), then in Bern as much as 5 and 24%, respectively, of the measurements would indicate moderate ID (Po0.0001). Similar comparison for Brussels would display much more marked differences with 23 and 57%, respectively, despite the absence of any value of iodine excretion below 0.39 mmol/24h (or o50 mgi/ 24h). Considering the other thresholds of 0.59 (75 mg I/24h), 0.79 (100 mg I/24h), 0.98 (125 mg I/24h) and 1.18 mmol iodine (150 mg I/24h),thedeviationswerealsoimpressive(Table2).As a whole, data calculated for spot samples did not strikingly differ from those obtained for 24 h collections. No single value with significant iodine excess (42.36 mmol I/24h or 4300 mg I/24h) was observed in the 110 collections of 24 h urine. Discussion Nowadays, quantification of urinary iodine is accepted as a valuable estimation of the iodine intake. As WHO recommends the use of the unit I/dL (WHO, 1994, 2001) instead of the former I/cr (Follis et al, 1962), comparison of both with the rather cumbersome and sometimes unreliable measure of I/24h is essential. Provided the 24 h urine is properly collected, it seems logical that the derived excretion of iodine should represent iodine intake more appropriately than concentration. Moreover, and in contrast to 24 h iodine excretion, a single spot of urine does not represent the mean value of different iodine concentrations in several mictions over a 24 h period. For this reason I/24h is the gold standard of iodine supply, despite the fact that up to 10% of the daily iodine intake may leave the human body by fecal or perspiration losses (Vought et al, 1963). To our knowledge, this is the first methodological study on iodine based on repeated 24 h collections and spot samples in adult subjects allowing direct comparison between I/24h, I/ cr and I/L. Considering I/24h as a gold standard, the additional variables creatinine and volume account for a highly significant difference in iodine concentration when compared to iodine excretion in our study samples (Tables 1 and 2). In all our study subjects median urinary iodine (I/cr or I/ L) grossly underestimated iodine excretion compared to I/ 24h. For our 110 collections of urine, the underestimation averaged 30 35%. The percentages of values below threshold levels corresponding to no, marginal, mild, intermediate, moderate or severe ID were all significantly higher with I/cr or I/L as compared to I/24h (Table 2). This observation applies to both I/cr and I/L regardless of the sampling method (spot samples or 24 h collections). Surprisingly, the difference was more marked for thresholds corresponding to mild and intermediate ID, that is, for situations where a decision about iodine supplementation might still be debated. Under these conditions, an appropriate evaluation of the iodine intake should consider not only the absolute iodine level but also the kind of samples that have been obtained and how the data are expressed. Our results of a significant overestimation of the frequencies of iodine-deficient samples with I/cr and I/L as compared to I/24h in a well-nourished population can certainly not be extrapolated to situations in developing countries, where it was demonstrated that using the I/cr ratio may result in a marked overestimation of the iodine intake (Bourdoux et al, 1985). Such overestimation results from low creatinine output because of low protein intake (Follis et al, 1962; Bourdoux et al, 1985; Bourdoux, 1988). Indeed, creatinine excretion is significantly higher in industrialized countries with high proteoenergetic intakes than in populations from developing countries particularly when they have protein energy malnutrition (Bourdoux et al, 1985). I/cr and I/L in spot samples and in 24 h collections did not differ significantly. However, a highly significant difference was observed between I/cr or I/L in spot samples or 24 h collections and I/24h in 24 h collections. For evident reasons of feasibility, spot samples are used in most studies on iodine supply as well in industrialized countries as in developing countries (Follis et al, 1962; Bourdoux, 1988; Bürgi et al, 1999; Als et al, 2003). The fact that all spot samples were obtained at the beginning of each 24 h collection may represent a methodological bias. A highly significant 1185

6 1186 circadian rhythm in urinary iodine concentration (I/L) with nadirs in early morning and peaks in the evening has indeed recently been demonstrated (Als et al, 2000). However, the beginning of 24 h collections took place at different moments of the day so that such bias is unlikely. Collecting urine during a period of 24 h is not applicable to unreliable subjects and patients or under difficult technical conditions, that is, mainly in the field. There, only spot samples are easily available and only concentrations can be measured. The question, which of the concentration terms: I/cr or I/L is then preferable, depends on technical and financial aspects. The term I/L (I/dL), nowadays recommended by WHO, presents the advantage of the lowest cost, the best technical feasibility and the absence of creatinine variability linked to age, sex, muscle mass, diseases including notably hyper- and hypothyroidism, and ph conditions (Fuller & Elia, 1988; Spierto et al, 1997). This effect of ph on urine creatinine stability would be of special importance to those who routinely use acid for stabilizing their urine specimens. We prefer not to acidify urine samples. Moreover, the specific influence of overweight on urinary iodine data is as yet unknown. Overweight subjects possibly represent a group with heterogeneous urinary iodine data, depending on qualitative as well as on quantitative aspects of their usual diet. None of our study subjects was overweight. On the other hand, the term I/cr may be more adequate from the biological point of view. However, a normal renal function is a prerequisite and malnutrition does affect glomerular filtration. Considering data from our study subjects all together, the coefficient of variation for creatinine in individuals varied from 4 to 20% (mean CV 12%) confirming previous observations in humans (Bourdoux et al, 1985). Even if creatinine excretion seems to be the most reproducible parameter, our data again question the assumption that creatinine is actually constant from day to day in a given individual. Moreover, in 24 h collections I/cr did not perform better than I/L; both grossly underestimated the iodine intake with similar amplitude (Table 1). In many countries in the world, ID is severe and therefore represents a public health problem more serious than in Belgium and Switzerland. In such countries, however, mainly epidemiologic and not metabolic studies have been performed. Only few metabolic data are available. In Congo (ex-zaire) for instance, it has been shown that urinary iodine expressed as I/cr represents a serious bias because of malnutrition or undernutrition (Bourdoux et al, 1985). In Kivu, Congo (ex-zaire), I/cr and I/24h are definitely not identical (Bourdoux et al, 1998). These punctual data are in agreement with the present findings. Future metabolic studies from countries with different nutritional situations are needed. The median urinary level of 0.79 mmol I/L (10 mg I/dL) recommended by WHO as the minimal target value for the iodine intake has not been established on data resulting from metabolic studies but has been decided on an arbitrary basis (WHO, 1994, 2001). This level was selected on the plea that it very roughly corresponded to an iodine excretion of 1.18 mmol/day (150 mg iodine/day). Considering that this mode of presentation grossly underestimates the iodine intake given by I/24h, one can speculate as to whether such minimum level of urinary iodine concentration should be maintained or reduced. Our data were sampled during the months of November and December. Following recently published knowledge and in conditions of a temperate climate, urinary iodine concentration is higher in winter than in summer months in milk-drinking subjects (Als et al, 2003). Owing to iodine enrichment of industrial cattle-food, iodine content of milk is higher in winter, when cattle is kept in stables, than in summer, when cattle weeds outside (Als et al, 2003). Despite our sampling in winter, iodine supply was nevertheless mildly to intermediately insufficient in Bern and Brussels, if the recommended daily intake of 1.18 mmol/day (150 mg I/ day) was considered as an optimum threshold (Delange, 1993). In Brussels, former data on iodine in adults, children and pregnant women had shown even less optimistic results than the present ones (Beckers et al, 1993; Delange et al, 1997). The present improvement corroborated by data on iodine in other age groups (Bourdoux, 1998) could possibly be explained by modifications of dietary habits in Belgium. In Bern however, the situation is different. Iodine supply had been considered sufficient during the 1980s (up to mmol I/mmol cr or up to mg I/g cr), after iodide content in salt had been doubled from 10 mg KI/ kg (7.5 ppm iodine) to 20 mg KI/kg (15 ppm iodine) in 1980 (Bürgi et al, 1990; Bürgi, 1993). As a consequence, the prevalence of goiter in school children and in army recruits had fallen to very low rates (Bürgi, 1993). In the beginning of the 1990s, however, consideringfas was usual thenfthe unit I/cr alone, iodine supply was again described as mildly insufficient: mmol I/mmol cr or 87 mg I/g cr (Als et al, 1995; Wüthrich, 1995). The unstable iodine supply over time has been explained by changes in alimentary habits, depending progressively more on foreign industrial food preparations with uniodized salt (Als et al, 1995). If the criteria recommended by WHO are considered, then the present data are the sixth confirmation of a resurgence of mild ID since the beginning of the 1990s (Als et al, 1995; Solca & Gerber, 1996; Als et al, 1997, 2003; Brander, 1997). Fortunately, and despite this situation, no increase of thyroid volume in school children in several Swiss regions has been registered in (Truong et al, 1997; Bürgi et al, 1999), as well as in (Als et al, 2003). Of course, if childrenfand not adultsfwere selected for the Thyromobil study, it is because their sensitive thyroid glands would most promptly become goitrous in case of relevant ID. However, these results obtained in school children cannot be extrapolated to our adult subjects. In conclusion, our findings of highly significant differences between I/24h, I/cr and I/L not only in 24 h urine collections but also in spot samples are based on

7 two adequately nourished groups of subjects living in a temperate climate in industrialized countries. The observed deviations could even be larger in a general population, depending for instance on age (Konno et al, 1993). Any variable used in the standardization of iodine, be it creatinine or volume, introduces additional error. Noteworthy is the observation that I/cr and I/L inaccurately overestimated ID as compared to I/24h. Those differences have to be kept in mind when assessing the iodine status of a population or when comparing own data with results in the literature. Our results clearly indicate that I/24h, I/cr and I/L are definitely not equivalent. We then strongly advocate that on the basis of prospective metabolic studies, separate normal ranges be established for I/24h, I/cr and I/L in 24 h urine collections and in spot samples. Since our data clearly establish marked discrepancies between iodine excretion and the arbitrary thresholds of iodine concentration in terms of mmol/l(mg/l), there is an urgent need to re-evaluate these threshold levels that obviously underestimate the real iodine intake causing unnecessary or unjustified adjustments of iodine supplementation. Moreover, each centre repeatedly measuring iodine should stick to one favored unit. Switching blindly from one measure of iodine to another as concerns own as well as literature data would generate, as we have shown, numerical differences prejudicial to a follow-up of iodine prophylaxis. Provided independent metabolic studies confirm our findings, future reference ranges of I/L and I/cr might be defined at a level about one-third lower than I/24h. Future separate normal ranges will then mainly serve as references for already existing literature data and for comparison with data from other centers. For prospective studies with an epidemiological purposefin accordance with WHO recommendationsfthe preference should be given to the term I/L in spot urine samples, collected in a sufficient number of fasting subjects. For metabolic studies, I/24h remains the gold standard. Results should preferably be expressed in both molar and metric units. Acknowledgements This work was supported by grants from the Swiss National Foundation for Scientific Research, the Fondation Genevoise de Bienfaisance V. Rossi di Montelera and the Schüpbach Foundation. The Fonds de la Recherche Scientifique Médicale Belge (Convention n to P.B.) also supported it. Presented in part at the 24th annual meeting of the European Thyroid Association (ETA) in Munich, Germany, on 2 September 1997 (Als et al, 1997) and at the 12th IFCC European Congress of Clinical Chemistry in Basel, Switzerland, on 29 August 1997 (Als et al, 1997, The Official Publication of Medlab 97. Proceedings of the European Congress of Clinical Chemistry 1997, 285.) References Als C, Haldimann M, Bürgi E, Donati F, Gerber H & Zimmerli B (2003): Swiss pilot study of individual seasonal fluctuations of urinary iodine concentration over two years: is age-dependency linked to the major source of dietary iodine? Eur. J. Clin. Nutr. 57, Als C, Helbling A, Peter K, Haldimann M, Zimmerli B & Gerber H (2000): Urinary iodine concentration follows a circadian rhythm. A study with 3,023 spot urine samples in adults and children. J. Clin. Endocrinol. Metab. 85, Als C, Lauber K, Brander L, Lüscher D & Rösler H (1995): The instability of dietary iodine supply over time in an affluent society. Experientia (Basel) 6, Als C, Lüscher D, Gerber H, Brander L, Lauber K & Rösler H (1997): Warum besteht erneut und noch immer Jodmangel in der Region Bern? Schweiz. Med. 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