5 Systematic literature review and grading of evidence on the health effects of
5.4 Studies concerning mild to moderate iodine deficiency
5.4.2.6 Grading of thyroid function – schoolchildren
One cross-sectional study on iodine status and thyroid function in schoolchildren were identified. The study found elevated Tg and free triiodothyronine (T3) in schoolchildren with mild to moderate iodine deficiency. It is therefore concluded that the evidence is limited (no conclusion) that mild to moderate iodine deficiency in schoolchildren is associated with thyroid dysfunction.
Fertility and birth outcomes
Six prospective cohort studies measuring iodine status and fertility and birth outcomes were identified, all categorised as B.
In a prospective cohort study of pregnant women in US, the relationship between iodine status and pregnancy loss was measured in 329 women (Mills et al., 2019). The risk of loss was not elevated in the mildly, moderate or in the severe deficient group. Urinary iodine concentrations were in the deficiency range in 60% of the participants. The authors concluded that iodine deficiency at levels seen in many developed countries does not increase the risk of pregnancy loss.
A prospective study including 541 pregnant women was conducted in London, Leeds, and Manchester between 2004 and 2008 (Snart et al., 2019). The median UIC was 134 µg/L and was between 100 and 150 in all three cities. Less than 20% had UIC < 50 µg/L. UIC was not associated with pregnancy outcomes including birth weight, growth restriction (SGA) and spontaneous preterm birth.
91 In a prospective cohort study of women (n=501) planning a pregnancy in US (Mills et al., 2018), women with UIC <50 µg/L took significantly longer to become pregnant, experiencing a 46% decrease in probability of becoming pregnant over each cycle compared to women with UIC ≥100 μg/L (adequate iodine status in non-pregnant women). Women with UIC 50–
99 μg/L (the mildly deficient range group) had a smaller, non-significant increase in time to conception, suggesting that the risk, if any, is modest in this group.
In a prospective cohort study from UK (Torlinska et al., 2018) on pregnant women with median UIC of 95 µg/L (moderate iodine deficiency in pregnant women) (n=3524), no associations were found between iodine status and seven different measures on adverse pregnancy outcomes including birth weight and preterm birth.
In a cohort study from Thailand (Charoenratana et al., 2016) in pregnant women (n=410), UIC<150 µg/L was associated with increased rate of preterm birth and low birth weight. In addition, women with a UIC <100 µg/L (moderate iodine deficiency in pregnant women) had a significantly higher rate of foetal growth restriction.
In a cohort study from Spain (Alvarez-Pedrerol et al., 2009) in pregnant women (n=657), higher birth weight and lower risk of having a small for gestational age (SGA) new-born was reported, in mothers with UIC between 100-149 µg/L (mild iodine deficiency in pregnant women), compared to mother with UIC<50 µg/L (severe iodine deficiency in pregnant women).
92 Table 5.4.3-1 Overview of six prospective cohort studies included for assessment of mild to
moderate iodine deficiency in women and fertility and pregnancy/birth outcomes, all categorised as B (sorted by publication year and author).
Reference,
N, age Fertility, pregnancy and birth outcome
Spot urine samples at the first in-home interview prior to conception. Median UIC 170 µg/d. Iodine was measured as µg/L and creatinine was used either as an independent variable or to use the ratio of iodine to creatinine (µg/g creatinine). The exposure was grouped into normal status (≥150 µg/L, n=133 (40%)), mild deficiency (100-149 µg/L, n=52 (16%)), moderate deficiency (50-99 µg/L, n=74 (22.5%)) and severe deficiency (<50 µg/L, n=70 (21%)). was identified by a positive test and pregnancy loss identified by conversion to a negative test.
The women with UIC in the deficiency range did not have a different loss rate than those in the sufficientgroup.
Results were consistent in sensitivity analyses including creatinine adjustment or exclusion of women being treated for thyroid disease.
Snart et al., 2019 UK ST29
Urinary iodine concentration was measured in two spot urines, one in 15 gw and one in 20 gw. The mean UIC of both time points were the main exposure. Median UIC was 134 µg/L, (139 µg/L in Manchester, 130 µg/L in London and 116
No evidence of an association between UIC and birth weight centile, nor with odds of spontaneous preterm birth or SGA. Sensitivity analyses gave similar results (using creatinine adjusted UIC, µg/g Cr) and assessing iodine at the individual time points.
Mills et al., 2018, US ST19
Spot urine samples from 467 women as UIC and I/Cr-ratio.
Iodine status was sufficient (UIC≥100 μg/L) in 260
(55.7%), mildly deficient (50–99 μg/L) in 102 (21.8%),
moderately deficient (20–49 μg/L) in 97 (20.8%) and severely deficient (<20µg/l) in 8 (1.7%).
Median UIC was 112.8 µg/L in the entire population, 114.1 µg/L in those who became pregnant, 97.2 µg/L in those who did not become pregnant, and 113.6 µg/L in those who moderate to severe deficiency range, it took significantly longer time to become pregnant, and they had a 46% decrease in probability of becoming pregnant over each cycle compared to women who were iodine sufficient. The mildly deficient range group had a smaller, non-significant increase in time to conception, suggesting that the risk, if any, is modest in this group.
93
N, age Fertility, pregnancy and birth outcome
(v) preterm birth; (vi) mode of delivery; and (vii) birthweight
No associations were found between iodine status and adverse pregnancy outcomes.
Repeated urine samples, UIC in 1st (n=384), 2nd (n=325) and 3rd (n=221) trimesters. Median UIC 151 µg/L. The mean UIC of repeated measurements were categorised into ≥150 µg/L and
<150 µg/L. The study also report the findings on foetal growth restriction using a cut-off of 100 µg/L.
associated with increased risk of preterm birth and low birth weight.
UIC <100 µg/L was associated with higher risk of foetal growth restriction.
No association with the other outcomes.
UIC was categorised into five categories <50 µg/L, 50-99 µg/L, 100-149µg/L, 150-249
Women with 3rd trimester UICs between 100-149 µg/L had lower risk of having an SGA newborn than women with UICs below 50 µg/L (aOR (95%CI): 0.15 (0.03, 0.76). The newborns in this group also had higher mean birth weights.
Similar results were indicated for 1st trimester UICs, but did not reach statistical
significance.