Exploring salivary iodine concentration as a biomarker for iodine status and thyroid nodules in females from different water iodine areas: a cross-sectional study

Background It is unclear whether salivary iodine concentration (SIC) can assess iodine status in females from different water iodine regions. Objective Through a cross-sectional study, we explored the feasibility of SIC as a biomarker to assess iodine status in females and develop optimal cut-off values. Methods A total of 1991 females were analyzed in this cross-sectional study from the coastal iodine-deficient areas (CIDA), inland iodine-deficient areas (IIDA), iodine-adequate areas (IAA), iodine-excess areas (IEA), and iodine extra-high areas (IEHA). SIC, spot urine iodine concentration(SUIC), and daily total iodine intake (TII) were assessed, and ultrasonography was performed in all subjects. Results There was a positive correlation between SIC and SUIC (r = 0.67, 95% CI: 0.64-0.69, P < 0.001), and TII (r = 0.47, 95% CI: 0.43-0.50, P < 0.001). The prevalence of thyroid nodules (TN) showed an upward trend with SIC increasing (Z = -2.83, P for trend =0.005). The area under the receiver-operating characteristic (ROC) curve for SIC to assess iodine deficiency was 0.62 (95% CI: 0.60-0.65, P < 0.001) and 0.75 (95% CI: 0.73-0.77, P < 0.001) for iodine excess. The cut-off values were: SIC<93.32 μg/L, iodine deficiency; 93.32-224.60 μg/L, iodine adequacy; >224.60 μg/L, iodine excess. When SIC>224.60 μg/L, the odds ratio (OR) for UIC>300 μg/L, excessive TII, and the prevalence of TN were 6.44, 3.68, and 1.27 (95% CI: 4.98-8.31, 2.83-4.79, 1.02-1.56, respectively; P < 0.05); when SIC<93.32 μg/L, the OR for UIC<100 μg/L and insufficient TII were 2.34, and 1.94 (95% CI: 1.73-3.14, 1.33-2.83; P < 0.05). Conclusions Using SIC as a biomarker, females in CIDA exhibited mild iodine deficiency, those in IIDA and IAA demonstrated moderate iodine deficiency, and those in IEA and IEHA exhibited an excess of iodine, consistent with SUIC to assess iodine status. SIC can be used as a good biomarker to evaluate the iodine status in population.