Iodine status of euthyroid adults: A cross‐sectional, multicenter study

Background Iodine, an essential nutrient, is the most important trace element in thyroid hormone synthesis and maintenance of thyroid function. This study investigated the iodine nutrition status in healthy Chinese adults and assessed the relationship between urinary iodine concentration (UIC) and thyroid hormone levels. Methods A cross‐sectional, multicenter study was conducted between October 2017 and January 2018, with 1017 adults recruited from five cities in China. All subjects underwent thyroid ultrasonography, and only those with normal results were included in the study. UICs were measured by inductively coupled plasma mass spectroscopy and adjusted using urine creatinine levels. Thyroid hormone levels were measured using an automated immunoassay analyzer. Results The median UIC and adjusted UIC were 134.0 µg/L and 114.2 µg/g, respectively. UIC was not significantly different between males and females (P = 0.737). However, the adjusted UIC was significantly different between sexes (P < 0.001). The median UIC was higher than 100 µg/L. According to the World Health Organization criterion (100 µg/L), the total prevalence of iodine deficiency is 33.1% (n = 271). The prevalence rates of iodine deficiency in our study were 33.2% and 32.9% in males and females, respectively, and had no difference between sexes and among cities (P > 0.05). Serum thyroid‐stimulating hormone (TSH) levels increased when UIC increased. The Kruskal‐Wallis test showed no significant differences in free triiodothyronine, free thyroxine, and TSH, with different levels of UIC (all P > 0.05). Conclusions Chinese adults with normal thyroid structure have relatively sufficient iodine levels.

3. Took thyroid medication in the past 15 days; 4. Was a hospital inpatient or seriously ill during the previous 4 weeks; 5. Surgery in the past 6 months; 6. Female participants, pregnant, breastfeeding, or within 1 year after childbirth; 7. On a high-iodine diet or consumed seafood including kelp, sea fish, crab, shrimp, and shellfish in the past 3 days.
A total of 1017 apparently healthy participants aged 18-82 years were enrolled in this study. This study was approved by the Ethics

Committee of the Institute of Peking Union Medical College
Hospital. All participants studied were informed in writing of the intended use of their samples, and each participant provided written consent.

| Data collection and physical examination
Data including demographic characteristics and medical history were collected from a representative sample of the study via a standard questionnaire. Body weight was measured on a calibrated beam scale, and height was measured in triplicate. Body mass index (BMI) was calculated as body weight divided by the square of the height (kg/m 2 ). Blood pressure (BP) was measured three times after the participant rested quietly for at least 10 minutes, and the average of three measurements was used. Current smoking status was classified as a self-reported response of "yes" to the question "Do you smoke now?" We also evaluated the UIC distribution between intake iodine salt and noniodine salt among 693 subjects who selfreported a response to the question "do you consume iodine salt during breakfast, lunch, or dinner?" All participants underwent thyroid ultrasonography examination performed by trained technicians.

| Laboratory measurement
All subjects were advised to have a bland diet before blood testing.
Following overnight fasting, blood was drawn from the antecubital vein of the arm. Spot urine samples were also collected. Blood specimens were centrifuged at 3000 rpm/min for 10 minutes. All samples were sent to the laboratory and stored at −80°C until tested.
Calibration and quality controls (Lyphochek ® Control) were performed before the analyses to monitor instrument precision.
Measurements were performed according to the standard operation procedure. Instrument calibration and preventive maintenance were performed annually. We also participated in External Quality Assessments by the National Center for Clinical Laboratories and College of American Pathologists to guarantee the accuracy and reliability of results. UIC was measured by inductively coupled plasma mass spectroscopy. Urine creatinine was measured using Beckman AU 2700 Automatic Biochemical Analyzer. Serum lipoprotein, including total cholesterol (TC) and triglycerides (TG), and fasting blood glucose (FBG) were measured. Thyroid hormones including free triiodothyronine (FT3), FT4, and TSH were measured using Beckman DXI 800 chemiluminescent immunoassay. The reference range for FT3, FT4, and TSH were 2.5-3.9 pg/mL, 0.61-1.12 ng/dL, and 0.38-5.33 mIU/L respectively. The precision of FT3, FT4, and TSH measurements was assessed according to the Clinical Laboratory and Standard Institution EP-15A2 protocol. We previously used this method for measuring UIC. The results revealed that the inter-run coefficients of variation (CVs) and total CVs for urine iodine were 3.5%-6.7% and 3.9%-6.7%, respectively. The intra-and interassay coefficient of variation for FT3, FT4, and TSH were 5.4%-8% and 5.8%-7.6%, 4.1%-7.6% and 1.7%-6.9%, and 2.4%-3.9% and 2.2%-3.7%, respectively, which meet the clinical values.
All laboratories participating in the survey followed the same internal quality control program that was standardized by the Peking Union Medical College Hospital.

| Iodine status
The iodine status of subjects was assessed by median UIC based on the World Health Organization (WHO) recommendations. According to iodine nutrition epidemiologic criteria of WHO, a population's median UIC of <100, 100-199, 200-299, and ≥300 µg/L is each representative of insufficient, adequate, above requirements, and excessive iodine intake. In this study, UIC of enrolled subjects was classified by <100, 100-299, and ≥300 µg/L. Furthermore, the prevalence of iodine deficiency was defined as proportion of subjects with a UIC <100 µg/L. The UIC values were adjusted using urine creatinine. 9 Urine iodine can be expressed in a relationship with creatinine excretion (µg iodine/g creatinine) also called adjusted urine iodine concentration equation 1. 9 for data analysis. Normally distributed data were presented as mean and standard deviation (SD), while skewed data were expressed as median (percentiles). Categorical variables were presented as a number (percentile). Group differences of normally distributed values were compared using the t test or one-way ANOVA, and skewed data were compared using the Mann-Whitney U or Kruskal-Wallis test. Group differences of categorical variables were compared using the chi-square test. P < 0.05 was defined as statistically significant.

| Characteristics of participants
A total of 1017 adults from five cities were recruited for this study.
Subsequently, 198 subjects lacking complete information or urine iodine measurements were excluded. Ultimately, 819 subjects with complete information and UIC and urine creatinine measurements who met the inclusion criteria were used in the final analysis. Baseline characteristics of study subjects are shown in Table 1. Among 819 subjects, the average age, BMI, systolic blood pressure (SBP), and diastolic BP (DBP) were 41.3 ± 13.2 years, 23.3 ± 3.6 kg/m 2 , 122 mm Hg, and 76 mm Hg, respectively. One-way ANOVA showed that there were significant statistical differences in age, BMI, SBP, DBP, FBG, TG, TC, FT3, FT4, and TSH among different cities (P < 0.001).

| Iodine status of the population
The median UIC and adjusted UIC (P 25 , P 75 ) were 134.0

| UIC and thyroid function
The median concentrations of FT3, FT4, and TSH in the subjects were 3.36 pg/mL, 0.91 ng/dL, and 1.95 mIU/L, respectively. Table 3 shows changes in serum FT3, FT4, and TSH concentrations accord-

| Relation of UIC and other indicators
Among the enrolled subjects, 712 answered the question "Are you smoking now?" and the median UIC of these subjects was 137.8 The correlations between UIC and indicators using Spearman correlation analysis are shown in Table 4. There was a statistically significant negative correlation between UIC and age, where UIC decreased as age increased. The relationship between UIC and adjusted UIC showed a positive correlation (P < 0.05).

| D ISCUSS I ON
This cross-sectional study includes the latest survey to date examining the iodine status, and the association between UIC and TA B L E 2 Iodine status of the study population in China classified by the World Health Organization criteria N (%)

100-299 µg/L N (%) ≥300 µg/L N (%)
Age-group (years) However, most studies used only UIC to estimate the iodine status of a population. 3,14 This study demonstrated that the median UIC varied with age but not with geographic location. A recent study with subjects aged 20 years and older also reported that the median UIC decreased according to age, supporting our data. 12 We also found that the median adjusted UIC varied by age and geographic location although we did not find various regularities between UIC and adjusted UIC.
The iodine nutrition status of the Chinese population has been suggested to be sufficient in several studies. 3 The iodine nutritional status in the adult population of the Shandong province was reported to have a median UIC of 248.5 µg/L. 15 In this study, the Chinese population had above sufficient levels of iodine.
The UIC in this study was lower than in the previous study. It might be that the UIC was affected by many factors, such as the place of residence (inland, seashore), eating habits, and economic development. However, the overall UIC showed the iodine levels to be sufficient in China. In this study, the subgroups with a higher UIC were associated with a higher median serum TSH, but not with statistical significance, and a relationship between UIC and FT3 or FT4 levels was not evident. Due to the large interindividual variation in the ability of the thyroid to adapt, thyroid hormones, including FT3, FT4, and TSH, are not considered sensitive indicators of the population iodine status. 10 Evidence suggests that levels of thyroid hormones will remain within normal range in mild iodine deficiency, while the hormone levels will fall outside the normal ranges only in cases of severe iodine deficiency. 1 Several studies reported the relationship between UIC and thyroid functions. 5,14,16 A Korean study reported that the serum TSH and FT4 levels showed statistically significant as UIC. 12 The prevalence of clinical hypothyroidism, subclinical hypothyroidism, and positive thyroid antibodies, assessed with UIC, was significantly higher in individuals with more than adequate iodine intake, than in individuals with adequate iodine intake. 14 Although no statistical significance was observed between UIC and thyroid hormones in our study, it is important to control the iodine nutrition intake.
This study demonstrated that the median UIC in smokers was higher than in nonsmokers, but not with statistical significance.
Kang et al reported that active smokers had significantly lower iodine levels than passive smokers and nonsmokers. 17 Regardless of smoking status, both groups were associated with decreasing serum TSH levels, which might be related to lower urinary iodine levels. 17 It was unclear whether smoking decreased the urinary iodine levels until now.
A strength of this study is that it is the latest study to report an association between UIC and its relationship with thyroid hormones in a Chinese population whose thyroid ultrasonography tests were normal. Additionally, we used urine creatinine adjusted UIC to evaluate the iodine status to ensure the appropriate evaluation of iodine nutritional status. This study still has several limitations. An important limitation is the lack of information on iodine intake via medications, or other sources. Lastly, we used UIC to assess the iodine status of a population. Spot urine sample UIC has been well documented as a suitable indicator for assessment of a population's iodine status. Therefore, currently it is the most suitable indicator to assess iodine status in a population-based study.
In conclusion, the iodine status of apparently healthy Chinese adults was found to be sufficient. However, salt iodization is still necessary to prevent iodine deficiency.

ACK N OWLED G M ENTS
The authors express their sincere gratitude to all participants and workers who contributed to this study. Funding support for this study was provided by the National Natural Science Foundation of China (81702060).

CO N FLI C T O F I NTE R E S T
The authors have no conflict of interests.

AUTH O R CO NTR I B UTI O N S
DCW, SLY, HLL, SWX, QC, and LQ performed the experiments.
DCW, SLY, YCY, and XQC analyzed the data. DCW, SLY, and HLL wrote the article. DCW, HLL, LQ, and YCY revised the article. DW and SY contributed equally to this article. All the authors have accepted responsibility for the entire contents of this article and approved its submission.