Ten Consecutive Days of Spot Urine Sampling for Stable Estimation of Individual Urinary Iodine Excretion

Twenty-four-hour urinary iodine excretion is the reference 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.

This study aimed to determine the minimum number of spot urine samples required to provide a stable estimate relative to the 14-d estimated 24-h urinary iodine excretion (e24hUIE) reference and to identify factors associated with repeated spot urine-derived e24hUIE.

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 1 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-d 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, within-individual coefficient of variation (CV), and intraclass correlation coefficient.

A total of 108 participants (mean age: 45.2 ± 17.6 y; 68 females) were analyzed. The median UIC was 112.7 μg/L [interquartile range (IQR): 60.4-197.0], and the median 14-d median e24hUIE was 175.9 (IQR: 122.8-231.8) μg/d, with a mean within-individual CV of 59.8%. Compared with the 14-d reference, generalized estimating equation analysis using robust sandwich standard errors showed that systematic differences in log-transformed median e24hUIE became nonsignificant from 9 d onward after false discovery rate (FDR) correction [10 d: β = 0.011 (95% confidence interval: -0.006, 0.028); FDR-adjusted P = 0.264]. Using 10 consecutive samples, 79.6% of participants achieved an estimation precision within ±10%. Participants aged >50 y had lower iodine status, whereas obesity and consumption of kelp/seaweed or seafood were associated with higher e24hUIE.

Collecting 10 consecutive days of spot urine samples provides a stable spot urine-derived estimate relative to the 14-d e24hUIE reference.