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

Abstract

Iodine deficiency may cause thyroid dysfunction. The iodine intake in a population is measured by urinary iodine concentration (UIC) in spot samples or 24-h urinary iodine excretion (24UIE). 24UIE is considered the gold standard and may be estimated using an equation including UIC, urinary creatinine concentration, sex and age (e24UIE). The aims of this study were to evaluate the preferable timing of UIC when using this equation and assess the variability of UIE. Sixty healthy non-smoking women (n 31) and men (n 29) were included in Gothenburg, Sweden. Twelve urine samples were collected at six fixed times on two separate days. Variability was calculated for UIC, 24UIE, e24UIE, iodine excretion per hour (iHr) and UIC adjusted for creatinine and specific gravity. Median 24UIE was 156 µg/24 h and the median UIC (all spot samples) was 104 µg/l. UIC (P < 0·001), 24UIE (P = 0·001) and e24UIE (P < 0·001) were significantly higher in men. e24UIE was relatively similar to 24UIE. However, when e24UIE was calculated from UIC in the first void, it was about 15 % lower than 24UIE (P < 0·001). iHr was lowest in the morning and highest in the afternoon. Median iHr was higher in men (7·4 v. 5·3 µg/h, P < 0·001). The variability of UIE was higher within individuals than between individuals. This study suggests that most time points for estimation of individual 24UIE are appropriate, but they should preferably not be collected in the first void.

Keywords: 24-h urinary iodine excretion; Estimated 24-h urinary iodine excretion; Iodine; Sampling time; Urinary iodine concentration; Urine sampling; Variability.

Fig. 1.

Geometric mean for iodine concentration spot urine samples (UIC as µg/l), UIC adjusted for urinary creatinine (UIC/Cr as µg/g creatinine), iodine excretion per hour (iHr as µg/h) and estimated 24-h urinary iodine excretion (e24UIE as µg/24 h). The geometric mean of 24-h urinary iodine excretion (24UIE as µg/24 h) is represented by a line for comparison. Day 2 outlier data for one subject excluded. The geometric mean is calculated from individual 2-d means.

Fig. 2.

Scatter plots showing positive correlation between (a) geometric mean of iodine excretion over 24-h sampling (GM24UIE) and geometric mean of estimated 24-h excretion based on an evening sample (GMe24UIE 22.00 hours) (r _s = 0·743) and (b) gm24uie and geometric mean of estimated 24-h excretion based on a first void sample (GMe24UIE first void) (r _s = 0·745). The regression lines display the association between the two variables and without an intercept. Day 2 outlier data for one subject excluded. The geometric mean is calculated from individual 2-d means.