Establishing reference intervals for urine and serum iodine levels: A nationwide multicenter study of a euthyroid Chinese population

Context Urinary iodine (UI) is commonly used for evaluating iodine status, whereas serum iodine (SI) is more closely correlated with bioavailable iodine. However, no reliable reference intervals (RIs) for clinical use are available. We aimed to establish RIs for SI, UI, and a ratio of UI to urinary creatinine (U-Cre) applicable to the Chinese population. Methods: This multicenter cross-sectional study enrolled 930 apparently healthy adults from six representative cities in China (Beijing, Dongying, Guiyang, Urumqi, Shenzhen, and Qiqihar) in 2017. Thyroid ultrasonography and thyroid function tests, including antithyroid antibody tests, were performed to exclude individuals with latent thyroid diseases. An iodine intake-related questionnaire survey was performed. SI and UI were measured using inductively coupled plasma-mass spectrometry. Possible influencing factors of iodine levels were evaluated using multiple regression analysis. Results: Post-exclusion, the final analysis included 894 individuals. Seafood intake frequency was positively correlated with SI (standardized partial regression coefficient = 0.23) but not with UI and UI/U-Cre. SI was positively correlated with serum TT4 (Spearman correlation coefficient: 0.40), TT3 (0.23), and FT4 (0.18). SI and UI showed no age- or sex-specific variations. Significantly higher UI/U-Cre values were observed in Qiqihar than in Beijing, Guizhou, and Shenzhen. Shenzhen showed the lowest UI levels among all evaluated cities. With application of latent abnormal values exclusion procedurere, the RIs for SI, UI, and UI/U-Cre in the population were 36.0-79.3 mu g/L, 19-385 mu g/L, 22-450 mu g/g, respectively. Conclusions: We established RIs for UI and SI among healthy Chinese individuals with no thyroid nodule or dysfunction.