Urinary iodine excretion

Countries affected by iodine deficiency require to develop national programmes to assess the extent and severity of the problem. Once an IDD control programme is initiated monitoring and evaluation are required. There are three major components needed to meet this goal, namely determination of thyroid size and goitre prevalence, the determination of urinary iodine excretion, and the measurement of thyroid function, including serum TSH levels. 

Urinary Iodine Excretion (UIE) provides the best single measurement of iodine intake of the population and Should be used for initial and follow up assessment. For epidemiological studies, population and not individual levels are is required. To achieve this 40 casual samples from a particular group can be collected (may be collected from schoolchildren at the same time as the goiter is assessed). The values are expressed as a median. Median UIE in the population below 100 pg/1 indicate iodine deficiency. Thus median UIE 10 pg/1 means no deficiency, 50-99 pg/1 indicates mild, 20 9 pg/1 moderate, and <20 pg/1 severe IDD. 

In 99 of 9320 newborns TSH concentrations were above the reference range (20 mU/ml) on the fifth day of life, but between the 10th and 21st day all these infants had normal TSH concentrations and normal thyroid function (38). In 76 of the newborns with hyperthyrotropinemia, urinary iodine excretion was significantly raised (above 16 pg/ml). Most of them were born in obstetric departments where iodophores were routinely used for disinfection during labor. 

Fisher, D. A. (1989). Upper limit of iodine in infant formulas. /. Njdr. 119,1865-1868-Fumee, C. A., Pfann, C. A., West, C-, Kaat, E, Heidc, D-, and Hautvasi, J. (1995). New model for describing urinary iodine excretion Its use fur comparing different oral preparations of iodized oil. Am, J, CHh, Nutr. 61,1257-1262. 

Hampel R, Goedalia A and Zollnee H et al. (2000) Continous rise of urinary iodine excretion and drop in thyroid gland size among adolescents in Mecklenburg-Westpommemfrom 1993 to 1997. Exp Clin Endocrinol Diabetes 108 197-201. 

Figure 13.10 Correlation between iodine content of tap water in 41 towns in Denmark and the average 24 h urinary iodine excretion in young men in each town. Source Pedersen et al., (1999) reproduced with permission.

Figure 35.3 The figure shows iodine excretion in groups with different use of traditionai Greeniandic food items, inciuding sea mammais, fish and wiid fowi. The differences in food choice influence urinary iodine excretion, with a graduai decrease towards the group that iives mainiy on imported foods. Adapted from Andersen eta/., (2005).

In a substudy of MoBa, including pregnant women grouped as users or nonusers of vitamin and mineral supplements, median urinary iodine excretion per 24 h was 190p,g and 110p,g, respectively (Brantsaeter et al., 2007). In the same study, dietary iodine intake was calculated by a food-frequency questionnaire and a food diary. The dietary intake of iodine among nonsupplement users was below the recommendations, whereas the dietary iodine intake of supplement users was above the recommendations. This demonstrates that supplements may contribute considerably to the total dietary iodine intake. 

Urinary iodine excretion (yg/i or yg iodine iodine/g creatininef deficiency stage Assessment of popuiations iodine intake/nutrition status... 

One female with history of iodine-containing X-ray after 01.01.02 and urinary iodine excretion >2000rig iodine/g creatinine excluded. 

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. 

Figure 44.2 is based on the same data as (Andersen et ai, 2001), and illustrates that individual mean values vary (Andersen et ai, 2003). The difference between individual mean urinary iodine concentrations is highly significant (Kruskal—Wallis test p < 0.001 for all variables), compared to that of TSH in serum (Andersen et al., 2001). This is consistent with other findings of marked differences in urinary iodine excretion between individuals (Rasmussen et ai, 1999 Busnardo et ai, 2006). 

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...

Table 44.2 shows measures of dispersion of urinary iodine concentration, and includes estimated 24 h urinary iodine excretion both for individual samples and for the average of 12 monthly samples. 

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. 

Urinary iodine excretion reflects dietary iodine intake over a limited period prior to urine sampling, and generally 90% of the ingested iodine is excreted in the urine (Keating and Albert, 1949 Dworkin et al., 1966 PaUardo et al, 1979 Hays, 1993 Hurrell, 1997 Jahreis etal., 2001). 

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 ... 

If the study provides estimated 24 h urinary iodine excretion, the precision range, or reliability of estimates, is approximately 20% higher, as can be read from Table 44.4. 

When planning a study of iodine nutrition, the number of urine samples needed to describe the iodine excretion level in a population can be read from Table 44.4 (Andersen et al., 2007a). If a precision range of 10% is aimed for, still with 95% confidence, about 125 spot urine samples are needed. Using the estimated 24 h urine iodine excretion reduced the number of samples needed by 20%, i.e., to 100 samples. A precision range of 5% required around 500 spot urine samples, while estimating 24h urinary iodine excretion reduced the required number of spot urine samples to around 400, as can be read from Table 44.4 (Andersen et al., 2007a). 

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