Urinary iodine excretion variation

Variation in urinary iodine excretion is important because it influences the study of iodine nutrition. Urinary iodine excretion exhibits large variations. The components of variation include preanalytical, analytical and biological variations. Biological variation consists of between- and within-individual variations, and can be broken down into chronobiological variation, i.e., diurnal and seasonal variations, and variations related to dilution, dietary peculiarities and supplement use. This is important for the evaluation and planning of studies of iodine nutrition, and it can be calculated that 500 urine samples depict population iodine nutrition level within 5%, while 125 urine samples are required for a value of 10%. Estimating 24h urinary iodine excretion lowers variation, and consequently the number of urine samples needed is reduced by around 20%. Similarly, it can be calculated that less than 10—15 urine samples from an individual may be misleading. 

This chapter aims to provide a description of these variations in urinary iodine excretion, some components and determinants of this variation, and the importance of variation for the interpretation of measurements of iodine excretion used to describe iodine nutrition in groups and in individuals. 

Table 44.1 shows the analytical goals for urinary iodine excretion, and includes analytical goals for thyroid function tests for comparison, calculated from variation in a study using a routine laboratory setting (Andersen et ai, 2001). The analytical variation is less important for urinary iodine, because the biological variation is high in this set-up. 

Urinary iodine excretion displays very wide variations compared with most other biological analytes. 

Figure 44.4 gives an impression of the two components of biological variation in urinary iodine excretion within-individual variation (vertical) and between-individual variation (horizontal). 

Table 44.2 Variation in urinary iodine concentrations and estimated 24 h urinary iodine excretion in individuai sampies and in average annuai vaiues over 1 year...

This is a rather simple model to describe the components of variation that can be obtained from most statistical programmes using analysis of variance (ANOVA) facilities, and the interpretation here is that the biological variation in urinary iodine excretion is around 2.5 times larger than the variation between individuals. 

Variation in Urinary Iodine Excretion and Studies of Iodine Nutrition... 

In the study of variation in urinary iodine excretion, an estimated 24 h urinary iodine excretion was calculated from age-, gender- and ethnic-specific creatinine excretions (Kampmann et al., 1974 Kesteloot and Joossens, 1996). The reduction in variation in iodine excretion is seen in Figure 44.5. 

The variation in estimated 24 h urinary iodine excretion is clearly lower than the variation in iodine concentration in spot urine samples. This improves the accuracy of calculations based on variation in iodine excretion (Andersen et al., 2007a). 

Figure 44.5 Variation in urinary iodine excretion expressed as crude urinary iodine content ( rg/l) and as 24 h urinary iodine excretion estimated from creatinine excretion in an age- and gender-matched group ( rg/24h).

The variation in urinary iodine excretion affects the reliability of estimates of population iodine nutrition. Low urinary iodine is seen in iodine-replete individuals due to random variation (Andersen et al., 2001). However, a high number of samples increases the reliability of the estimates of iodine excretion in a population, but what is the reh-ability of a study including a certain number of spot urine samples from a population ... 

In the smdy of variation in more than one variable, variations can be correlated. Such associations may be analyzed (Feldt-Rasmussen et al, 1989). The association between iodine excretion and serum TSH was studied in 15 healthy men in an area with mild-to-moderate iodine deficiency (Andersen et al., 2001). The association differed between individuals, and when individuals were grouped according to urinary iodine excretion levels, a negative correlation was found only in individuals with an iodine excretion below 50iig/24h (Figure 44.6). 

Variations in urinary iodine portray variations in iodine intake. However, huge variations in sample iodine content do not reflect similar variations in the iodine nutrition in that population. This is important for risk estimation in the evaluation of population iodine intake, and has important implications for studies of iodine nutrition. Furthermore, the reliability of studies of iodine nutrition and the number of urine samples needed to assess iodine excretion level can be calculated. 

Adjusting urinary iodine excretion for age-, gender-and ethnic-specific creatinine excretions markedly reduces variation in urinary iodine. 

Figure 119.3 Geographical variations in urinary iodine excretion in Denmark before iodine fortification of salt. Median urinary iodine excretion among inhabitants of various Danish cities before iodine fortification of salt, and the estimated number of people living in areas with different levels of urinary iodine excretion. Values were compiled from different studies of urinary iodine excretion, or estimated from measurements of groundwater iodine content. Geographical variation in iodine intake in Denmark is mostly determined by differences in groundwater iodine content. Pedersen etal., (1999) Rasmussen etal., (2000).

In 2003 the prevalence of goiter, as evaluated by ultrasound, was 4.3% and the occurrence of visible goiter was 0.4%. The median urinary iodine of a representative group of 809 children in this national study was 192p,g/l, with a variation from 162 p,g/l to 246p,g/l. The relative number of cases with urinary iodine excretion less than 100p,g/l was 7.7%. These results indicated that IDD had been reduced below the level of public health significance. 

There are various well-established biomarkers of intake and/or nutritional status of numerous food components (Margetts and Nelson, 1997 Wilett, 1998). In the case of iodine, a good measure of iodine intake is urinary excretion, because most (more than 90%) of iodine ingested is excreted in urine. Thus, the urinary iodine concentration, even in casual urine samples, is a good marker of iodine nutrition. Urinary iodine concentration varies with fluid intake, so these values have limited use for casual samples from an individual, but they are well-suited for assessing a population group, because individual variations tend to average out. 

Variation in population minary iodine excretion is often described as the number of individuals with minary iodine excretion below a certain value. Such a number is, however, a parameter in the description of the variance characteristic to the population, and not a fraction of the population with low urinary iodine, as interpreted in quite a number of studies. 

Variable fluid intake causes variable volumes of urine. Around 90% of dietary iodine is excreted in the urine, and variable urine volumes cause variable dilution of the iodine excreted in urine, and thus in the concentration of iodine in urine. If the variation in dilution is corrected, the variation in urinary iodine content is lowered. This can be achieved by correcting for an analyte that is excreted in parallel with urine volume. 

Table 44.4 Number of spot urine samples needed to be 95% oonfident of being within a speoified range for crude urinary iodine con-oentration and for estimated 24 hour urinary iodine exoretion oaloulated from the variation in iodine excretion among heaithy men undertaking daily lives (Andersen et al., 2007b)...

The dietary associations were generally weak, but the true associations may have been masked by measurement error including that arising ffom intraindividual variation in spot measurements of urinary iodine. An individuals iodine concentration varies substantially day-to-day, due to the combined effects of circadian patterns of iodine excretion (Als et at, 2000), rapid renal clearance of dietary iodine following meals and inconsistency in the iodine content of food. We were able to average measures for children in the pre-intervention surveys who participated... 

We specifically asked the doctors in each medical school to select patients who had not received any kind of medicine containing iodine or medical exams which could lead to iodine overload. It should be noted however that the patients were free to have side dishes in addition to the regular hospital diet. Therefore, such wide variations in urinary excretion may be related to iodine supplements coming from the diet. 

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