Urinary Iodine in 24 h Is Associated with Natriuresis and Is Better Reflected by an Afternoon Sample

Abstract

Aims: 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. Methods: 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. Results: 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). Conclusions: 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.

Iodine status of Queensland children and the associations with diet and thyroid function

“…This observation is consistent with the knowledge that ingested iodine appears in the urine after an approximate 4-5 hour delay (56). Another cross-sectional study involving 60 volunteers, concluded that UIC from spot samples collected in the afternoon, between lunch and dinner time, have a stronger correlation to urinary iodine concentrations determined by 24hr collection compared to first morning spot samples (57). A 24 hour sample collection of urinary iodine in Denmark also supported the theory of variable iodine excretion throughout the day (58).…”

“…There is strong evidence to suggest that collecting afternoon samples to determine UIC will provide a better representation of iodine status (56,57).…”

“…For this reason, UIC variation will replicate the variation in the diet throughout the day and between days. Studies report that in both adults and children UIC shows a significant circadian rhythm, being lowest in the morning and peaking in the early afternoon (19,56,57,127). (19,20,56,57,127).…”

“…Studies report that in both adults and children UIC shows a significant circadian rhythm, being lowest in the morning and peaking in the early afternoon (19,56,57,127). (19,20,56,57,127). This could misrepresent the distribution of iodine status within the population, which may inadvertently impact the evaluation of any iodine nutrition intervention that may be in process.…”

“…These studies also reinforce the benefit in collecting at least one extra sample from each participant to provide a more accurate reflection of the actual iodine status distribution within a population. Although these differences were determined to be statistically insignificant, the pattern of including both lows and peaks in daily UIC increases the possibility of representing actual iodine status (56,57). This is not to suggest, however, that collecting four UIC samples is representative of individual iodine status, as it is advised that at least 10 repeated urine samples are required to ascertain individual iodine status (129).…”