Correlation between iodine intake and thyroid function in subjects with normal thyroid function

Excessive iodine intake is known to induce hypothyroidism in people who have underlying thyroid disorders. However, few studies have been performed on subjects with normal thyroid function without a history of autoimmune thyroid disease. We hypothesized that high iodine intake may cause a subtle change in thyroid function even in subjects with normal thyroid function. We analyzed 337 subjects (64 men and 273 women; mean age, 49 years) who showed normal levels of thyroid peroxidase antibodies (TPO-Ab) and thyroglobulin antibodies (Tg-Ab) by measuring the urinary iodine excretion, free T4 (FT4), and thyroid-stimulating hormone (TSH). The results showed urinary iodine excretion had negative correlation with FT4 (γ = -0.11, p = 0.043) and showed a positive trend with TSH (γ = 0.10, p = 0.068). We found that 61.7% of subjects had circulating TPO-Ab within normal reference range. In all subjects, TPO-Ab levels were negatively correlated with FT4 (γ = -0.17, p = 0.002) and positively with TSH (γ = 0.13, p = 0.021). In conclusion, high iodine intake can negatively affect thyroid hormone levels in subjects with normal thyroid function. Population-based study will be helpful for further clarification.