Urinary iodine levels and thyroid disorder prevalence in the adult population of China: a large-scale population-based cross-sectional study

In the past decade, the incidence of thyroid disorders has been steadily increasing, emerging as a prominent public health concern. Consequently, there is a growing interest in understanding the association between iodine nutritional status and thyroid disorders. We selected 13,487 adults (aged 18-69 years) from the baseline population of Zhejiang Environmental Health Cohort (ZEHC) research. Serum thyroid function indicators, urinary iodine concentration (UIC), and urinary creatinine were measured and an ultrasonography of the thyroid were systematically assessed. Urinary iodine/creatinine ratio (UI/Cr) was calculated to mitigate hydration bias. The median of UIC and UI/Cr were 158.2 (IQR: 97.0-250.5) µg/L and 113.4 (IQR: 69.5-178.4) µg/g, respectively. Excessive iodine intake (UIC ≥ 300 µg/L) increased the risk of subclinical hypothyroidism (adjusted OR 1.451, 95% CI 1.252-1.681), while insufficient iodine intake (UIC < 100 µg/L) reduced this risk (adjusted OR 0.831, 95% CI 0.716-0.965). Conversely, insufficient iodine intake (UIC < 100 µg/L) was associated with elevated thyroid nodule incidence (adjusted OR 1.196, 95% CI 1.099-1.301). After creatinine adjustment, the risk of subclinical hypothyroidism was higher in high (quartile 4) UI/Cr level (adjusted OR 1.520, 95% CI 1.334-1.732), and participants with low (quartile 1) UI/Cr level exhibit a lower risk of subclinical hypothyroidism (adjusted OR 0.624, 95% CI 0.523-0.744). Participants with low (quartile 1) UI/Cr level had a significant increase in the incidence of thyroid nodule (adjusted OR 1.315, 95% CI 1.203-1.437). This large-scale population-based study found that higher iodine level was associated with an increased risk of subclinical hypothyroidism, while lower iodine level was associated with an increased incidence of thyroid nodules.