Association Between Iodine Status and Prevalence of Hypothyroidism, Autoimmune Thyroiditis, and Thyroid Nodule: a Cross-Sectional Study in Shandong Province, China

In this study, the aim was to investigate the correlation between varying levels of urinary iodine concentration (UIC) in adults and the occurrence of thyroid diseases, with the additional objective of determining the optimal iodine status level for adults. A cross-sectional study was conducted on adults from six areas with different drinking water iodine concentrations (WIC) without eating iodized salt in Heze and Jining counties, Shandong Province, China. A total of 1336 adults were included in this study, and drinking water samples, blood samples, urine samples, thyroid ultrasound, and a questionnaire were collected. UIC, free triiodothyronine (FT3), free thyroid hormone (FT4), thyroid-stimulating hormone (TSH), thyroid peroxidase antibody (TPOAb) and thyroglobulin antibody (TgAb) were detected. There were no significant differences in the detection rates of hypothyroidism and thyroid autoimmunity (TAI) among the different median UIC groups (UIC < 100 μg/L, 100–199 μg/L, 200–299 μg/L, ≥ 300 μg/L). However, the detection rates of hypothyroidism were higher in the UIC < 100 μg/L group (16.67%) and the UIC ≥ 300 μg/L group (16.51%) compared to the other groups. The detection rate of TAI increased as UIC levels increased. The detection rate of thyroid nodule (TN) in the UIC < 100 μg/L group was significantly higher than that in the UIC 200–299 μg/L UIC group (χ = 10.814, P = 0.001). After adjusting confounding factors, it was found that low UIC (< 100 μg/L) was a risk factor for TN (OR 1.83, 95% CI [1.04–3.22]). Meanwhile, there no statistical difference between UIC 200 and 299 μg/L and UIC 100 and199 μg/L for OR of hypothyroidism, TAI, and TN. This study identified associations between different UIC levels and the prevalence of thyroid disorders, with low UIC (< 100 μg/L) posing a risk for TN, and the detection rate of TN and hypothyroidism was the lowest in UIC (200–299 μg/L) group. Therefore, the acceptable UIC range of ‘adequate’ iodine intake among adults can be widened from 100–199 µg/L to 100–299 µg/L.