Ten consecutive days of spot urine sampling provide stable estimations of individual urinary iodine excretion.

BACKGROUND: Twenty-four-hour urinary iodine excretion is the gold standard for assessing iodine status. Although median urinary iodine concentration (UIC) from spot urine samples is widely used for population-level assessment, its reliability for individual evaluation remains uncertain. OBJECTIVE: This study aimed to determine the minimum number of spot urine samples required to provide a stable estimate relative to the 14-day estimated 24-hour urinary iodine excretion (e24hUIE) reference and to identify factors associated with repeated spot urine-derived e24hUIE. METHODS: In this repeated-measures observational study, healthy adults were recruited from Taipei Veterans General Hospital and National Yang Ming Chiao Tung University, Taiwan (July, 2022 - November, 2023). Each participant provided one random spot urine sample daily for 14 consecutive days. The e24hUIE was derived from spot samples using a creatinine-based prediction equation, with the 14-day median e24hUIE serving as the internal repeated-measures reference. Estimation stability and precision across varying sample numbers were evaluated using the mean of absolute median difference (MMD), within-individual coefficient of variation (CV), and intraclass correlation coefficient. RESULTS: A total of 108 participants (mean age 45.2 ± 17.6 years; 68 females) were analyzed. The median UIC was 112.7 μg/L (IQR 60.4-197.0), and the median 14-day median e24hUIE was 175.9 (122.8-231.8) μg/day, with a mean within-individual CV of 59.8%. Compared with the 14-day reference,GEE analysis using robust sandwich standard errors showed that systematic differences in log-transformed median e24hUIE became nonsignificant from 9 days onward after FDR correction (10 days: β = 0.011 [95% CI -0.006, 0.028]; FDR-adjusted p = 0.264). Using 10 consecutive samples, 79.6% of participants achieved an estimation precision within ±10%. Participants older than 50 years had lower iodine status, whereas obesity and consumption of kelp/seaweed or seafood were associated with higher e24hUIE. CONCLUSIONS: Collecting 10 consecutive days of spot urine samples provides a stable spot urine-derived estimate relative to the 14-day e24hUIE reference.