Challenges in the Evaluation of Urinary Iodine Status in Pregnancy: The Importance of Iodine Supplement Intake and Time of Sampling

Abstract

Objectives: Median urinary iodine concentration (UIC) is the recommended method to evaluate iodine status in pregnancy, but several factors may challenge the interpretation of the results. We evaluated UIC in pregnant women according to (1) sampling in the hospital versus at home, (2) time of the most recent iodine supplement intake prior to sampling, and (3) members of their household. Study Design: Danish cross-sectional study in the year 2012. Pregnant women (n = 158), their male partners (n = 157) and children (n = 51) provided a questionnaire with detailed information on iodine supplement intake and a spot urine sample obtained in the hospital and/or at home for measurement of UIC and urinary creatinine concentration. Results: In the pregnant women providing a urine sample both in the hospital and at home (n = 66),individual UIC (p = 0.002) and urinary creatinine concentration (p = 0.042), but not estimated 24-hour urinary iodine excretion (p = 0.79), were higher when sampling was at home. Median UIC was dependent on the time of the most recent iodine supplement intake prior to sampling [same day (n = 79): 150 µg/l (95% CI 131-181 µg/l), the day before (n = 51): 105 µg/l (78-131 µg/l), several days ago/non-user (n = 28): 70 µg/l (56-94 µg/l), p < 0.001]. The pattern was similar in the male partners. Apart from a more frequent iodine supplement intake in pregnancy (87.3% vs. partners 15.9%), no systematic differences were observed in urinary measurements between the pregnant women and their partners. Conclusions: Time of spot urine sampling and time span from iodine supplement intake to spot urine sampling should be considered when evaluating urinary iodine status in pregnancy.

Iodine Supplementation in Pregnancy and the Dilemma of Ambiguous Recommendations

“…However, it is important to be aware that a change in diet, including a change in fluid intake, may have occurred on this specific day. In our study of pregnant women [16], we collected spot urine samples from a group of pregnant women both on the day they visited the hospital for obstetric ultrasound and another day at home. The median UIC was 84 µg/l in the hospital and 133 µg/l at home.…”

“…We also measured urinary creatinine concentration in the Danish pregnant women investigated [16], and when the urinary creatinine concentration was used to estimate 24-hour urinary iodine excretion in these women, no difference was observed between sampling in the hospital and at home. Thus, the difference in UIC was mostly caused by differences in fluid intake [16]. Some obstetric clinics directly recommend excessive fluid intake to pregnant women before an obstetric ultrasound to allow better visualization of the fetus.…”

“…Thus, a spot UIC would depend much on whether the urine is collected before or some hours after the daily iodine supplement intake. Figure 2 illustrates how the timing of urine sampling in relation to supplement intake affected estimated 24-hour urinary iodine excretion in our Danish study [16]. …”

“…The p value is the result of the Kruskal-Wallis test (same day vs. the day before vs. several days ago/nonuser). Data are from Andersen et al [16]. …”

“…In our Danish study, we found no difference between the median UIC in pregnant women, their male partners and children when urine sampling was performed at home under similar conditions (fig. 3) [16]; however, iodine supplement use was much more frequent among pregnant women than among male partners and children [16]. Other investigators have reported different results, with a higher median UIC in children than in pregnant women [20,21].…”