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

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 h urinary iodine excretion (UIE). Twenty-six participants were categorised into low-iodine (<i>n</i> 17) and high-iodine (<i>n</i> 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 <i>v</i>. 27·89 µg/g, <i>P</i> = 0·03; post-lunch [1-PL]: 71·03 <i>v</i>. 27·4 µg/g, <i>P</i> = 0·003; post-dinner [1-PD]: 114·13 <i>v</i>. 31·58 µg/g, <i>P</i> = 0·002) and Day-2 (2-PB: 81·86 <i>v</i>. 26·51 µg/g, <i>P</i> = 0·013; 2-PL: 54·56 <i>v</i>. 18·83 µg/g, <i>P</i> < 0·001; 2-PD: 38·2 <i>v</i>. 18·79 µg/g, <i>P</i> = 0·043), whereas UIC only differed significantly post-dinner on Day-1 (156·15 <i>v</i>. 36·63 µg/l, <i>P</i> = 0·009). SI/P also showed stronger correlation with 24 h UIE (1-PB: <i>r</i> = 0·65, <i>P</i> = 0·001; 1-PL: <i>r</i> = 0·70, <i>P</i> < 0·001; 1-PD: <i>r</i> = 0·67, <i>P</i> < 0·001; 2-PB: <i>r</i> = 0·70, <i>P</i> < 0·001; 2-PL: <i>r</i> = 0·65, <i>P</i> = 0·001; 2-PD: <i>r</i> = 0·50, <i>P</i> = 0·01) compared with UIC (1-PB: <i>r</i> = 0·49, <i>P</i> = 0·011; 1-PL: <i>r</i> = 0·38, <i>P</i> = 0·055; 1-PD: <i>r</i> = 0·58, <i>P</i> = 0·002; 2-PB: <i>r</i> = 0·68, <i>P</i> < 0·001; 2-PL: <i>r</i> = 0·52, <i>P</i> = 0·007; 2-PD: <i>r</i> = 0·44, <i>P</i> = 0·027). 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.