Prolonged Exposure to Elevated Iodine Levels in Drinking Water Is Associated With the Occurrence of Autoimmune Thyroid Disorders in Adults: Findings From a Case-Control Study Conducted in Shandong Province, China.

Background: Excessive iodine intake is associated with an increased risk of thyroid autoimmunity. However, the relationship between prolonged exposure to iodine levels exceeding 100 μg/L in drinking water and the occurrence of autoimmune thyroid disorders (AITDs) remains uncertain. The objective of the present study was to assess whether elevated iodine levels exceeding 100 μg/L in drinking water are a risk factor for AITD. Methods: We conducted a case-control study at a hospital, enrolling 668 adults. We measured serum thyroid peroxidase antibody (TPOAb), thyroglobulin antibody (TgAb), urinary iodine concentration (UIC), water iodine concentration (WIC), and serum cytokine concentrations-including interleukin-1β (IL-1β), interleukin-6 (IL-6), interferon-γ (IFN-γ), and tumor necrosis factor-α (TNF-α). Results: The study demonstrated that individuals with water iodine levels > 100 μg/L had a significantly higher risk of developing AITD compared with those in the 10-100 μg/L group (OR = 2.076, p < 0.001). Furthermore, a family history of thyroid disorders (OR = 4.035, p < 0.001) and higher education levels (specifically college education compared to primary school; OR = 2.608, p=0.016) were associated with an increased risk of AITD. Conversely, regular consumption of freshwater fish was correlated with a lower risk of developing AITD (at least once per week vs. hardly eat, OR = 0.472, p=0.009; at least once per month vs. hardly eat, OR = 0.693, p=0.042). Additionally, IL-6 levels in the case group were significantly higher than those in the control group. Conclusions: The case-control study demonstrated a significant association between the development of AITD and prolonged exposure to elevated iodine levels (above 100 μg/L) in water. AITD was found to be associated with several other factors. Risk factors for AITD include a family history of thyroid disorders and higher educational attainment. Additionally, the consumption of freshwater fish was identified as a protective factor. Identifying and understanding these significant risk and protective factors for AITD development are critical, and effective strategies should be developed and implemented for prevention and intervention targeting at-risk individuals.