Correlation Between Salivary and Urinary Iodine Indicators in Lactating Women: A Cross-Sectional Study

Iodine is essential for infant growth and neurodevelopment. To assess iodine nutritional status in populations or individuals a 24-hour urinary iodine concentration (24 h-UIC) is a reliable biomarker but relatively time-consuming and inconvenient method, therefore, more simple alternatives are required. This study examined the correlation between salivary iodine concentration (SIC) and commonly used iodine-related indicators, that is, 24 h-UIC and breast milk iodine concentration (BMIC), and explored the potential of SIC as a biomarker in lactating women. This study recruited 29 healthy lactating women in Luoyang City, Henan Province, China. Over five consecutive days, 24-hour urine samples were collected daily, while saliva and breast milk samples were collected once a day. Inductively coupled plasma mass spectrometry (ICP-MS) was used to determine iodine concentrations in urine, saliva and breast milk. Statistical analyses included Spearman correlation analysis to assess associations between SIC and urinary iodine-related measurements. The median 24 h-UIC was 114.8 µg/L, indicating adequate iodine nutrition in this study population (median UIC < 100 µg/L). SIC showed significant positive correlations with 24 h-UIC and 24-hour urinary iodine excretion (24 h-UIE) (r = 0.689 and 0.460, respectively; P < 0.001), whereas no significant correlation was observed between SIC and BMIC (r = 0.047, P > 0.05). When participants were grouped according to 24 h-UIC (< 100 µg/L vs. ≥100 µg/L), higher SIC levels were observed in the group with higher 24 h-UIC (SIC: 163.4 vs. 109.2 µg/L). The intraindividual variability of SIC was lower than inter-individual variability (18.5 vs. 35.2%), suggesting relatively stable within-subject measurements over repeated sampling. The positive correlation between SIC and 24 h-UIC or 24 h-UIE suggests that SIC may serve as a convenient non-invasive indicator associated with urinary iodine-related measurements in lactating women. However, the observed relationships are exploratory, and further validation in larger and more diverse populations is needed.