Interpreting urinary iodine concentration: effects of urine dilution and collection timing.

Objectives: In population studies, iodine intake estimation relies on median urinary iodine concentration (UIC). However, interpreting UIC measurements can be challenging.

Methods: In our study, we included 772 adult participants from three groups: nationally representative gender-mixed, women of reproductive age, and pregnant women. We measured UIC and urinary creatinine (U-Cr) to calculate the iodine-to-creatinine ratio (I/Cr). U-Cr cut-off value of 0.226 g/L was used for differentiation between diluted and undiluted urine samples. After excluding samples below this cut-off, new median UIC and I/Cr ratios were calculated. We additionally evaluated the influence of urine sample collection time on UIC.

Results: Median UICs were 91.8 µg/L for nationally representative group, 58.3 µg/L for women of reproductive age, and 74.9 µg/L for pregnant women, while I/Cr ratios were 91.7, 102.0, and 159.2 µg/g, respectively. After implementing U-Cr cut-off and excluding all data where U-Cr was below cut-off, new median values were 93.4, 76.3, and 95.4 µg/L for UICs, and 88.6, 88.8, and 128.7 µg/g for I/Cr ratios, respectively. In women of reproductive age, median UIC was significantly lower in urine samples collected after 9:30 and after 12:00 as compared to samples collected before 9.30 (53.4, 57.8, and 97.3 μg/L, respectively).

Conclusions: UIC results should be interpreted with caution, considering urine dilution and sample collection timing. U-Cr measurement should be included in population-based iodine intake studies, with corrections applied especially for pregnant women and younger adults, for whom morning is best for single-spot samples.