Serum iodine reference ranges and their correlations with urinary iodine and thyroid function: A systematic review and meta-analysis

Abstract

Background: Serum iodine mainly exists in the form of iodine and iodide ions in thyroid hormones and does not change immediately due to variations in external factors. It is an important indicator reflecting iodine metabolism and actual iodine levels in the body. Given the significant differences in serum iodine among different countries and populations, the standard range of SIC has not yet reached a consensus. To the best of our knowledge, this systematic review and meta-analysis are the first to explore the serum iodine nutritional status of different populations across the country and its correlation with thyroid function. Objective: This meta-analysis aimed to assess reference ranges of serum iodine concentration (SIC) and the potential correlations among SIC, urinary iodine concentration, and thyroid function in various populations globally, providing a reference for the assessment of individual iodine nutritional status. Methods: This study was conducted by searching multiple databases, including PubMed, Web of Science, and China National Knowledge Infrastructure, to gather relevant studies on SIC. Two researchers independently screened the literature and extracted and synthesized the data. The data were then analyzed using Stata software to generate pooled estimates of SIC with 95% confidence intervals (CIs) using forest plots. Results: A total of 48 eligible studies were included, yielding a mean SIC of 88.68 μg/L (95% CI: 84.70-92.65 μg/L). The range of SIC reported across studies was 23.92-183.50 μg/L. Among these, most studies focused on the correlations between SIC and free thyroxine (FT4) (81.0%) and between SIC and FT3 (72.7%). Conclusion: This meta-analysis highlights the importance of SIC as a valuable indicator of individual iodine nutritional status. The findings suggest that SIC is influenced by regional, temporal, and methodological variations, emphasizing the need for standardized testing and reference values. Moreover, the observed associations between SIC and thyroid function markers underscore its clinical relevance in monitoring iodine-related health outcomes. Establishing consistent measurement protocols and population-specific reference ranges will enhance the accuracy of iodine status assessments and support more effective public health interventions. Relevance for Patients: This systematic review provides a preliminary synthesis of currently non-standardized SIC reference ranges. Future large-scale, multicenter studies on SIC testing are needed to establish reference values for assessing individual iodine nutritional status, thereby enabling scientifically informed and precision-based iodine supplementation.

Keywords

Population variability / Iodine nutritional status / Serum iodine / Medical reference values / Thyroid functions

Funding

This research was supported by the Non-communicable Chronic Diseases-National Science and Technology Major Project (grant numbers: 2024ZD0537900; 2024ZD0537907), the National Natural Science Foundation of China (grant number: 82373698), and the Key Research and Development Program (Innovation Base) of Heilongjiang Province (grant number: GY2024JD0040).

Conflict of interest

The authors declare that they have no competing of interests.

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