Correlation Between Iodine Intake and Thyroid Function in Subjects with Normal Thyroid Function

Abstract

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.

Longitudinal evaluation of thyroid autoimmunity and function in pregnant Korean women

“…In their study of thyroid antibody negative Japanese pregnant women (gestational age 7-15 weeks), Orito et al found that median urinary iodine excretion during the first trimester was extremely high (328 μg/L), TSH values were positively correlated with urinary iodine, and that 1.2% of the study population had a serum TSH > 4.90 mIU/L while still having a normal FT4 [18]. It is worth noting that adult Koreans consume a diet rich in iodine [19][20][21], but it is not known what effect (if any) this would have on the reference intervals established in the present study. Ultimately, the variation of data across published reports underscores the importance of using method-and population-specific reference intervals.…”

“…Iodine nutrition status was not assessed in this study as it has been previously reported that the adult Korean population consumes a diet rich in iodine (including seaweed and salt-water fish), and may in fact consume excessive iodine [19][20][21]. Due to the difficulty in obtaining agematched longitudinal samples for non-pregnant women (MizMedi is a maternal health specialty hospital), reference intervals and thyroid autoantibody status were not assessed in this population.…”

Effect of maternal excessive iodine intake on neurodevelopment and cognitive function in rat offspring

“…The results of some studies regarding excess iodine suggested that high iodine intake was associated with increased incidence of hypothyroidism [19,20], while others found that it led to hyperthyroidism [21,22]. These different results may be related to the duration of high iodine exposure.…”

Iodine and thyroid function

“…However, adverse effects of excess iodine were not apparent. In a study of 337 healthy South Korean adults, UI excretion had a weakly negative correlation with free T4 and showed a positive trend with TSH, whereas their levels of free T4 and TSH were within the normal ranges48). Iodine excess did not directly influence the risk of goiter in 69 Korean prepubertal children49).…”