An Inverse Relationship Between Iodine Intake and Thyroid Antibodies: A National Cross-Sectional Survey in Mainland China.

ABSTRACT

Background: Iodine intake is associated with thyroid autoimmunity. In this study, we evaluated the changes in thyroid autoimmunity after 20 years of universal salt iodization (USI) in China. Methods: A total of 78,470 subjects (18 years or older) from 31 provincial regions of mainland China participated in the study. Serum thyroid peroxidase antibody (TPOAb), thyroglobulin antibody (TgAb), TSH receptor antibody, thyrotropin (TSH), and urinary iodine concentration (UIC) were measured. Results: Positive TPOAb and TgAb were detected in 10.19% [CI 9.80-10.59] and 9.70% [CI 9.28-10.13] of the subjects, respectively. The prevalence of positive isolated TPOAb (i-TPOAb), positive isolated TgAb (i-TgAb), and double positive TPOAb and TgAb (d-Ab) was 4.52%, 4.16%, and 5.94%, respectively. The prevalence of thyroid antibody positivity was the highest in the iodine-deficient (UIC <100 µg/L) groups. The prevalence of i-TPOAb was inversely associated with more than adequate iodine intake (MAI) and excessive iodine intake (EI); the odds ratio (OR) was 0.89 [CI 0.81-0.98] for MAI and 0.90 [CI 0.81-0.99] for EI. We observed that i-TgAb, like i-TPOAb, was a high-risk factor for subnormal TSH levels (OR = 3.64 [CI 2.62-5.05]) and elevated TSH levels (OR = 1.62 [CI 1.49-1.77]). The prevalence of thyroid antibody positivity varied among five ethnic groups. Conclusions: After two decades of USI, the prevalence of thyroid antibody positivity has remained low. MAI and EI had an inverse relationship with TPOAb positivity, which reveals that UIC between 100 and 299 µg/L is optimal and safe for thyroid autoimmunity. These conclusions need to be confirmed in a follow-up study because this study was a cross-sectional study.