Is the urinary iodine/Creatinine ratio applicable to assess individual iodine status in Chinese adults? Comparison of iodine estimates from 24-hour urine and timed-spot urine samples in different periods of the day

Zhuan Liu Fujian Medical University A liated First Quanzhou Hospital Yixuan Lin Fujian Medical University Jiani Wu Fujian Center for Disease Control and Prevention Diqun Chen Fujian Center for Disease Control and Prevention Xiaoyan Wu Fujian Center for Disease Control and Prevention Ying Lan Fujian Center for Disease Control and Prevention Zhihui Chen (  zhihuichen@fjcdc.com.cn ) Fujian Center for Disease Control and Prevention https://orcid.org/0000-0002-1375-8254


Methods:
We collected 24-hour, spot and fasting urine in five periods of the day to estimate 23 24-hour UIE by the six different prediction equations. We compared the estimated 24 creatinine-corrected UIC to the collected 24-hour UIE and identified the most suitable 25 equations in each period of the day. 26 Results: Among the six different prediction equations, the equation of Kawasaki T was the 27 best to estimate the 24-hour UIE by fasting urine among Chinese adults. Among the five 28 periods of time, the equation of Knudsen N was the best to estimate the 24-hour UIE except 29 the morning period.
Timed-spot urine 34 35 Background 36 Iodine, as a key micronutrient for thyroid hormone synthesis [1], plays an essential role 37 in metabolism. It is estimated that around 1.88 billion people globally are still at risk of 38 insufficient iodine intake [2], and therefore iodine deficiency is regarded as a public health 39 issue in both developing and developed countries [2][3], especially in Eastern Mediterranean, 40 Asian, African and most of the Eastern European countries [2,4]. Due to the limited kinds of 41 iodine rich food [5], WHO introduced a salt iodization program to improve iodine status at the 42 populational level as a way of lowering the risk of iodine insufficiency [6]. China, as well as 43 most other countries [7][8], has considerably improved this issue by the universal salt 44 iodization program [9]. 45 In order to identify people at risk of iodine deficiency, it is important to monitor iodine 46 status at both population and the individual level [10][11]. The amount of iodine collected at 47 the spot urine sample is usually presented as the urinary iodine concentration (UIC) or the 48 urinary iodine/creatinine concentration ratio [12]. UIC can be used to evaluate the population 49 iodine status [8,13], however, we still lack the uniform standard in the prediction of the 50 individual iodine status. Some researchers used serum iodine [14], serum thyroglobulin [15] 51 and 24-hour urine [16] to assess individual iodine status. This study compared the six 52 prediction equations in five periods of the day to the real 24-hour urine iodine excretion (UIE), 53 aiming for providing evidence of individual iodine status assessment.  Participants were given a uniform urine collection bag, a sterile plastic urine collection   69   tube with a handle for dipping, and clear instructions for 24-hour urine collection. Participants   70 were required to record the start and finish times. All the participants were informed about the 71 collection methods and announcements. The urine was firstly collected after the first urination 72 in the early morning; then it was collected till the first urination on the next morning, lasting 73 for 24 hours. Each voiding time was recorded. There were five periods of the day for urine 74 collection: morning (after discarding the first void -12:30), afternoon (12: 31-17:30), evening 75 (17:31-23:59), early morning (00:00-03:59), and fasting time (the first void collected the next 76 morning after the longest duration of sleep) [17]. 24-hour urinary creatinine excretion [18] 77 was used as quality control for 24-hour urine collection, and participants with urinary 78 creatinine excretion of less than 75 per cent were eliminated across genders and ages [19].  [20], and urinary creatinine concentration was assayed applying urinary creatinine alkaline 85 picric acid spectrophotometry (WS/T97-1996) [21]. 86 Equations of the estimated and measured 24-hour urinary iodine excretion 87 For spot urine and fasting urine in five periods of the day, the six prediction equations 88 were employed to estimate 24-hour UIE in the same individual. The equations in Table 1 (Table 3)    The Y-axis is the difference between log-transformed estimated 24-hour urinary iodine excretion 1 and 168 log-transformed measured 24-hour urine iodine excretion;

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The solid black line represents the bias, and the dashed line represents the 95% range of consistency for the 170 mean relative difference; 171 Upper limit: upper 95% limit of consistency; lower limit: lower 95% limit of consistency. 172 We also discovered that the bias of Estimated 24-h UIE 5 varied from -0. 16   little evidence to support spot urine as a way to reflect iodine intake over the course of 24 213 hours [27]. As a consequence, some researchers have proposed a variety of prediction 214 equations to estimate 24-hour UIE [18,[22][23][24][25]. Some researches showed that multiplying I/Cr 215 by estimated 24-hour urine creatinine could be used to estimate 24-hour UIE while others 216 could not [28]. 217 Moreover, UIE varies during the day, but it is controversial on the circadian pattern. 218 UIC which was at the lowest level in the morning showed an upward trend from the morning 219 to the evening; and spot urine in the afternoon can represent 24-hour UIE, according to Als C 220 and Vanacor R et al [29][30]. As for fasting urine, one article pointed out that the UIC in 221 fasting urine samples was 10% lower compared to non-fasting urine samples [31]. 222 Nonetheless, some investigations reported that UIC in spot samples do not have a similar 223 tendency during the day [32]. Similarly, the results presented in Figure 1 do not have an 224 upward trend for all the participants, conversely, each participant had a different tendency. 225 According to our findings, the six prediction equations may have the applicability in the 226 assessment of individual iodine status. We still needed to employ 24-hour urine volumes for 227 calculation when using Estimated 24-hour UIE 1 albeit the most periods of the day (except the 228 morning) showed the good consistency. In addition, there was a large difference in 24-hour 229 urine volume between male and female [33][34]  Ethics approval and consent to participate 294 All the participants were recruited following the informed consent and ethical review.

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Consent for publication 296 The authors consent to the publication of the data.

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Competing interests 298 The authors declare that they have no competing interests.