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A total of 600 adults were recruited for this study. After excluded 27 participants (omitting thyroid hormones value or without thyroid ultrasound exam), the effective samples consisted of 190, 190, and 193 individuals in the iodine excess, more-than-adequate iodine, and deficient iodine groups, respectively. The median urinary iodine (MUI) levels of the three groups were 991.42 μg/L, 241.29 μg/L, and 55.48 μg/L. The following iodine nutrition criteria are provided by the WHO[5]: deficient, MUI < 100 μg/L; adequate, 100 μg/L ≤ MUI ≤ 199 μg/L; more than adequate, 200 μg/L ≤ MUI ≤ 299 μg/L; excess, ≥ 300 μg/L. The cohort was divided into iodine excess, iodine more than adequate and iodine deficient iodine nutrition groups according to these criteria, as shown in Table 1. The median iodine concentration of the drinking water ranged from 460.20 μg/L to 629.90 μg/L for the three high-iodine water villages. The demographic characteristics of the participants are presented in Table 2. The age, gender and BMI index significantly differed among the three groups (P < 0.01).
Iodine Status Sample Size Urinary Median
(Interquartile) (μg/L)Tg Median
(Interquartile) (μg/L)Tg > 77 μg/L
N(%)Iodine excess group 190 991.42 (666.58, 1428.08) 22.27 (9.28-43.79)# 24 (12.63)△ Iodine more than adequate group 190 241.29 (159.30, 337.76) 9.73 (5.20-20.25)*# 4 (2.11)△★ Iodine deficient group 193 55.48 (35.19, 91.82) 15.77 (9.38-25.88)* 8 (4.15)★ Note. #Kruskal-Wallis test, P < 0.001;*Kruskal-Wallis test, P = 0.001; △χ2= 15.422, P < 0.001; ★χ2= 1.313, P = 0.252. Table 1. The Tg Level in Different Iodine Status Groups
Characteristics Iodine Excess Group (n) Iodine More than Adequate Group (n) Iodine Deficient Group (n) P Value Participants number 190 190 193 Gender Male 130 62 43 < 0.001 Female 60 128 150 Age (years) Mean ± SD 53.91 ± 13.27 56.65 ± 11.86 52.8 ± 14.76 0.009 ≤ 40 45 35 66 0.008 41-60 84 88 67 61- 61 67 60 BMI (kg/m2) 24.1 ± 3.26 25.58 ± 3.72 23.16 ± 4.05 < 0.001 Education level※ Illiteracy 15 32 20 < 0.001 Primary school 93 82 55 Junior high school 66 57 63 Senior high school 13 10 31 College 3 2 24 Smoking★ None 165 156 184 < 0.001 1-10 8 12 0 11-20 5 1 7 > 20 9 19 2 Alcohol consumption△ None 161 169 178 0.536 Light 12 10 9 Heavy 12 9 6 Note. ★missing 5, △missing 7, ※missing 7. Table 2. The Demographic Characteristics of the Participants
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The serum Tg levels among excessive, more than adequate, and deficient iodine group were significantly different [22.27 (9.28-43.79) μg/L, 9.73 (5.20-20.25) μg/L, and 15.77 (9.38-25.88) μg/L, respectively], P < 0.001. Specifically, compared with that of the more than adequate iodine group, the median serum Tg levels of both iodine excessive and deficient groups were significantly higher (P < 0.01), as shown in Table 1. Figure 1 illustrates that the Tg levels differ among populations with differing iodine nutrition.
Figure 1. Box plot showing the the distribution of Tg levels of the three iodine status groups. Box plot explanation: upper horizontal line of box, 75th percentile; lower horizontal line of box, 25th percentile; horizontal bar within box, median; upper horizontal bar outside box, 90th percentile; lower horizontal bar outside box, 10th percentile.
The proportions of subjects with a Tg level exceeding 77 μg/L were 12.63%, 2.11%, and 4.15% in the iodine excess, more-than-adequate and deficient groups, respectively. These differences were significant among all three groups (χ2= 20.136, P < 0.001), between the iodine excess and iodine more-than-adequate groups (χ2= 15.422, P < 0.001), and between the iodine excess and the iodine-deficient groups (χ2= 9.006, P = 0.003), but not between the iodine-deficient and the iodine more-than-adequate groups (χ2= 1.313, P = 0.252).
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The prevalence of hypothyroidism (overt hypothyroidism and subclinical hypothyroidism) was 45.71%, 11.52%, and 13.61% for the Tg-above-77-μg/L group, Tg-below-77-μg/L group and the total population, respectively. This prevalence significantly differed between the Tg-above-77-μg/L and the Tg-below-77-μg/L groups (Fisher' s exact test, P < 0.001). The prevalence of thyroid nodules was 42.86%, 18.22%, and 19.72% in the Tg-above-77-μg/L group, Tg-below-77-μg/L group and the total population, respectively. This prevalence significantly differed between the Tg-above-77-μg/L group and Tg-below-77-μg/L group (χ2= 12.604, P < 0.001). The prevalence of thyroid enlargement was 11.43%, 2.23%, and 2.79% in the Tg-above-77-μg/L group, Tg-below-77-μg/L group and the total population, respectively. This prevalence did not significantly differ between the Tg-above-77-μg/L group and the Tg-below-77-μg/L group (Fisher's exact test, P = 0.013 > 0.0125), as shown in Table 3.
Thyroid Disease Tg > 77 μg/L
n(%)
(n = 35)Tg ≤ 77 μg/L
n(%)
(n = 538)Total
(n = 573)Hypothyroidism* 16 (45.71)△ 62 (11.52)△ 78 (13.61) Hyperthyroidism# 0 7 (1.3) 7 (1.22) Thyroid nodule★ 15 (42.86)※ 98 (18.22)※ 113 (19.72) Thyroid enlargement 4 (11.43) & 12 (2.23) & 16 (2.79) Note. *Including subclinical hypothyroidism and overt hypothyroidism. #Including subclinical hyperthyroidism and overt hyperthyroidism. ★Including single thyroid nodule and multiple thyroid nodules. △Fisher's exact test, P < 0.001; ※χ2= 12.604, P < 0.001; & Fisher's exact test, P = 0.013 > 0.0125. Table 3. The Thyroid Disease Prevalence in Different Tg Level Groups
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The serum Tg levels were significantly higher in subjects with nodules [21.06 (12.04-42.54) μg/L than in subjects without nodules 13.95 (6.87-25.04) μg/L], in subjects with TPOAb negativity [16.21 (8.90-28.24) μg/L than in subjects with TPOAb positivity 6.2 (1.57-26.33) μg/L], in subjects with TgAb negativity [16.34 (9.12-29.87) μg/L than in subjects with TgAb positivity 5.71 (1.50-21.84) μg/L], and in subjects with goiters [20.13 (12.40-81.55) μg/L than in subjects without goiters 15.31 (7.48-27.57) μg/L]. A Spearman rank correlation analysis indicated that the effects of smoking and alcohol consumption habit on serum Tg level were not statistically substantial (Table 4).
Factors N Tg Log (Tg) P* Median (Interquartile Range) μg/L Mean ± SD Gender Male 165 14.73 (7.72-27.45) 1.209 ± 0.451 0.746 Female 408 15.62 (7.68-29.86) 1.195 ± 0.479 Thyroid enlargement Yes 16 20.13 (12.40-81.55) 1.438 ± 0.555 0.040 No 557 15.31 (7.48-27.57) 1.192 ± 0.467 Nodule Yes 113 21.06 (12.04-42.54) 1.368 ± 0.45 < 0.001 No 460 13.95 (6.87-25.04) 1.158 ± 0.467 TPOAb Positive 94 6.20 (1.57-26.33) 0.932 ± 0.623 < 0.001 Negative 479 16.21 (8.90-28.24) 1.251 ± 0.415 TgAb Positive 111 5.71 (1.50-21.84) 0.877 ± 0.575 < 0.001 Negative 462 16.34 (9.12-29.87) 1.278 ± 0.406 Spearman correlation (r) Smoking 568 -0.061 0.143 Drinking 566 -0.039 0.354 Age 573 0.008 0.857 Note. The P value are the results of t test or spearman correlation. Table 4. Univariate Analysis of Other Factors Except for Iodine Intake in Relation to Serum Tg
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The serum Tg level was significantly higher (P = 0.005) in subjects with hypothyroidism [32.18 (6.36-63.83) μg/L] than that in subjects with euthyroidism [14.84 (8.05-25.32)], and the serum Tg level of subjects with euthyroidism was higher (P = 0.039) than that of subjects with hyperthyroidism [1.66 (0.05-12.96) μg/L], as shown in Figure 2.
Figure 2. Box plot of serum Tg levels derived from euthyroidism, hypothyroidism and hyperthyroidism populations. Box plot explanation: upper horizontal line of box, 75th percentile; lower horizontal line of box, 25th percentile; horizontal bar within box, median; upper horizontal bar outside box, 90th percentile; lower horizontal bar outside box, 10th percentile.
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The results of a multivariate linear regression model analysis demonstrated that the iodine nutrition status significantly affected the serum Tg level. The serum Tg levels were significantly higher in the iodine excess group (β = 0.262, P < 0.001) and in the iodine deficient group (β = 0.115, P = 0.014) than in the iodine more-than-adequate group. In addition, some factors were directly related to the serum Tg level; after adjusting for gender, age and BMI, thyroid enlargement (β = 0.227, P = 0.047), thyroid nodule (β = 0.112, P = 0.017), hypothyroidism (β = 0.172, P = 0.002) and TgAb positivity (β = -0.361, P < 0.001) were significantly related to the serum Tg level, as shown in Table 5.
Variables β 95% CI P Male vs. Female -0.029 -0.107-0.049 0.462 Age (Year) 0.002 -0.001-0.004 0.21 BMI (kg/m2) -0.004 -0.014-0.005 0.376 TPOAb positivity vs. TPOAb negativity -0.104 -0.223-0.015 0.086 TgAb positivity vs. TgAb negativity -0.361 -0.47-0.251 < 0.001 Nodule vs. Without nodule 0.112 0.02-0.203 0.017 Thyroid enlargement vs. Without thyroid enlargement 0.227 0.003-0.451 0.047 Excess group vs. More than adequate group 0.262 0.176-0.349 < 0.001 Deficient group vs. More than adequate group 0.115 0.024-0.207 0.014 Hypothyroidism* vs. Without hypothyroidism 0.172 0.064-0.281 0.002 Hyperthyroidism#vs. Without hyperthyroidism -0.121 -0.454-0.212 0.476 Note. *TSH > 5.57 mIU/L, #TSH < 0.38 mIU/L. Table 5. The Multiple Linear Regression Analysis for Clarifying the Determinant Factors of Log (Tg) Level