Participants
Of the 1598 participants recruited in this study, 278 adults were excluded due to incomplete data. 1320 adults were included in the final analyses, including 610 males and 710 females (Fig. 1). The mean age, height, weight, BMI and BSA of subjects were 42.9 ± 16.2 years, 42.9 ± 16.2 cm, 69.2 ± 29.0 kg, 24.9 ± 9.70 kg/m2 and 1.74 ± 0.40 m2, respectively. Table 2 shows the baseline characteristics of subjects grouped by gender. There was no difference in age between male and female, and the height, weight, BMI and BSA of male were significantly higher than those of female.
Thyroid function and iodine nutrient levels
The median (IQR) of UIC, UIC/UCr, SIC, TSH in subjects were 155.8 (94.5, 211.1) μg/L, 138.1 (91.1, 207.6) μg/g, 70.6 (59.8, 83.9) μg/L, 2.02 (1.41, 2.95) mIU/L, respectively. The range of UIC, UIC/UCr, SIC, TSH were 20.5-864.2 μg/L, 12.7-2349 μg/g, 25.9-119.9 μg/L, 0.005-37.4 mIU/L, respectively. And the 90% reference range for UIC/UCr and SIC were 66.5-349.8 mg/g and 49.3-97.1 μg/L. Grouped by gender, The levels of UIC (P=0.003), SIC (P=0.04), TSH (P<0.001) and TgAb (P<0.001) in female were significantly higher than those in male. Although the UIC/UCr level in female was also higher than that in male, there was no significant difference (P=0.08). The TPOAb level in female was slightly lower than that in male, but the difference was not statistically significant (P=0.24). (Table 2).
Correlation between SIC and UIC, UIC/UCr and TSH
The correlations between SIC and UIC, UIC/UCr and TSH were shown in Fig. 2. SIC was positively correlated with UIC (r=0.22, P < 0.001) and UIC/UCr (r=0.34, P < 0.001), and the correlation coefficients were 0.22 and 0.34, respectively; There was also a positive correlation between SIC and TSH (r=0.12, P < 0.001).
Thyroid dysfunctions prevalence grouped by gender
Thyroid dysfunctions included overt hyperthyroidism, subclinical hyperthyroidism, overt hypothyroidism, subclinical hypothyroidism, TgAb positive and TPOAb positive. The prevalence of hypothyroidism (P=0.02), subclinical hypothyroidism (P=0.002), TgAb positive (P<0.001) and TPOAb (P<0.001) positive in female was significantly higher than that in male, they were 2.26% vs 0.65%, 7.05% vs 2.62%, 18.9% vs 5.89%, 14.8% vs 6.06%, respectively. The prevalence of overt hyperthyroidism and subclinical hyperthyroidism in male was slightly higher than that in female, but the difference was not statistically significant. (Table 3).
Prevalence of thyroid dysfunctions in different sex and serum levels
The prevalence of thyroid dysfunctions in different sex and serum iodine levels was shown in Table 4. The reference interval of 90% serum iodine was 49.3-97.1 μg/L, divided into 3 groups according to the reference interval of 90% serum iodine for statistical analysis. In male subjects, the prevalence of hyperthyroidism (6.06%) and overt hypothyroidism (3.03%) in the SIC≥97.1 μg/L group was significantly higher than that in the 49.3<SIC ≤ 97.1 μg/L Group. In female subjects, the prevalence of overt hypothyroidism (6.10%), TgAb positive (29.3%) and TPOAb (28.0%) positive in the SIC < 49.3 μg/L group was significantly higher than that in the 49.3 ≤ SIC ≤ 97.1 μg/L Group.
The association between serum iodine and thyroid dysfunctions grouped by gender
It was also analyzed according to the reference range of 90% of total serum iodine. In male subjects, subjects with SIC>97.1 μg/L had a higher risk of hyperthyroidism (OR=2.91, 95% CI: 1.38, 6.19) and overt hypothyroidism (OR=5.59, 95% CI: 1.88, 6.42). In female subjects, subjects with SIC< 49.3 μg/L had a higher risk of overt hypothyroidism (OR=2.18, 95% CI: 1.10, 4.06), TgAb positive (OR=1.97, 95% CI: 1.15, 3.32) and TPOAb positive(OR=2.48, 95% CI: 1.41, 4.26). (Table 5-6).