Urinary iodine in 24 h is associated with natriuresis and is better reflected by an afternoon sample

We aimed to assess the extent of variability in urinary iodine (UI) within a day, to determine the period of the day when UI was better associated with the UI in 24 h, and to study the relationship between UI and urinary sodium.

This cross-sectional study included 60 volunteers. Four urine samples were collected from each participant (A: from breakfast to lunch; B: from lunch to dinner; C: from dinner to bedtime, and D: from bedtime to breakfast) and were compared with the 24-hour sample (calculated from samples A-D ). UI, creatinine and Na(+) levels were measured in the partial samples and in the 24-hour sample.

The content of iodine and sodium in urine varied during the day (p < 0.001). The UI concentration from lunch to dinner was closest to the 24-hour UI concentration using the method of Bland and Altman. There were correlations between the UI content in the different periods of the day and in 24 h: A (r = 0.54; p < 0.000), B (r = 0.78; p < 0.000), C (r = 0.37; p = 0.004) and D (r = 0.77; p < 0.000). UI and urinary sodium content were strongly correlated in all periods of the day (samples A and B: r = 0.69, p < 0.000; sample C: r = 0.85, p < 0.000, and sample D: r = 0.78, p < 0.000).

There was a within-day variation in UI content, which was strongly associated with urinary sodium content. Iodine concentration in the afternoon urine sample better reflected the 24-hour UI concentration. Therefore, urine collected in the afternoon is probably the best to evaluate iodine sufficiency in subjects with similar dietary habits.