Urinary iodine excretion and optimal time point for sampling when estimating 24-h urinary iodine

Area of Science:

• Nutritional Science
• Endocrinology
• Public Health

Background:

• Iodine deficiency can lead to thyroid dysfunction.
• Population iodine intake is assessed via urinary iodine concentration (UIC) or 24-hour urinary iodine excretion (24UIE).
• 24UIE is the gold standard, often estimated using a formula (e24UIE) incorporating UIC, creatinine, sex, and age.

Purpose of the Study:

• To determine the optimal timing for collecting spot urine samples to estimate 24UIE.
• To assess the variability of urinary iodine excretion (UIE) within individuals.

Main Methods:

• Sixty healthy adults provided 12 urine samples over two days at six fixed times.
• Calculated variability for UIC, 24UIE, e24UIE, hourly iodine excretion (iHr), and creatinine/specific gravity-adjusted UIC.
• Compared estimated 24UIE (e24UIE) with measured 24UIE.

Main Results:

• Men exhibited significantly higher UIC, 24UIE, and e24UIE than women.
• e24UIE closely approximated 24UIE, except when calculated from the first void, which underestimated 24UIE by ~15%.
• Hourly iodine excretion (iHr) peaked in the afternoon and was higher in men; intra-individual variability exceeded inter-individual variability.

Conclusions:

• Most spot urine collection times are suitable for estimating individual 24UIE.
• First morning void samples should be avoided for accurate e24UIE calculations.
• Understanding UIE variability is crucial for accurate iodine status assessment.