Protein-corrected salivary iodide as a potential biomarker of individual iodine status: a proof-of-concept comparison with urinary iodine in groups with contrasting iodine intake.

Abstract Urinary iodine concentration (UIC) is the principal biomarker for assessing iodine status; however, it is subject to marked fluctuations and practical challenges. This proof-of-concept study evaluated protein-corrected salivary iodide (SI/P) as a potential alternative biomarker, comparing it with spot UIC in distinguishing between iodine-deficient and iodine-adequate individuals, assessing its responsiveness to short-term dietary iodine restriction, and exploring its correlation with 24-hour urinary iodine excretion (UIE). Twenty-six participants were categorised into low-iodine (n = 17) and high-iodine (n = 9) groups based on 24 h UIE collected on Day-1. Postprandial spot urine and unstimulated saliva samples were collected under habitual diet (Day-1) and low-iodine diet (Day-2). SI/P was significantly higher in the high-iodine group at all time points on both Day-1 (post-breakfast 1-PB: 61.28 vs. 27.89 µg/g, p = 0.03; post-lunch 1-PL: 71.03 vs. 27.4 µg/g, p = 0.003; post-dinner 1-PD: 114.13 vs. 31.58 µg/g, p = 0.002) and Day-2 (2-PB: 81.86 vs. 26.51 µg/g, p = 0.013; 2-PL: 54.56 vs. 18.83 µg/g, p < 0.001; 2-PD: 38.2 vs. 18.79 µg/g, p = 0.043), whereas UIC only differed significantly post-dinner on Day-1 (156.15 vs. 36.63 µg/L, p = 0.009). SI/P also showed stronger correlation with 24 h UIE (1-PB: r = 0.65, p = 0.001; 1-PL: r = 0.70, p < 0.001; 1-PD: r = 0.67, p < 0.001; 2-PB: r = 0.70, p < 0.001; 2-PL: r = 0.65, p = 0.001; 2-PD: r = 0.50, p = 0.01) compared with UIC (1-PB: r = 0.49, p = 0.011; 1-PL: r = 0.38, p = 0.055; 1-PD: r = 0.58, p = 0.002; 2-PB: r = 0.68, p < 0.001; 2-PL: r = 0.52, p = 0.007; 2-PD: r = 0.44, p = 0.027). Receiver operating characteristic analysis indicated that post-lunch SI/P achieved the highest accuracy (AUC = 0.891; 95%CI: 0.791–0.991; p < 0.0001). Unlike UIC, which is primarily suited for population-level monitoring, SI/P demonstrated stable performance irrespective of diet/sampling time, suggesting utility as a reliable, individual-level biomarker of iodine status.