A research team from Japan wanted to examine the gestational change of urinary iodine excretion in Japanese women from an iodine-sufficient area. They also looked at the effect of iodine status on maternal and child thyroid function. Their study was published in the Journal of Clinical Endocrinology and Metabolism in December 2011 as the articles “Iodine status of pregnant and postpartum Japanese women: effect of iodine intake on maternal and neonatal thyroid function in an iodine-sufficient area.”
The study included 934 women and 722 newborns. During each trimester of pregnancy, iodine and creatinine concentrations were taken in spot urine samples. These concentrations were also taken during the postpartum period at 34 days after delivery.
Researchers also measured serum thyroperoxidase antibody and thyroglobulin antibody, TSH, and free T4 during each trimester.
In the children, researchers measured neonatal TSH 4 days after their birth.
The study showed that, during pregnancy, the overall median urinary iodine concentration (UIC) was 219.0 μg/L. That was higher than in women who had already delivered—135.0 μg/L.
16.1% of the pregnant women had low UIC (less than 100 μg/L); 22.2% of them had high UIC (greater than 500 μg/L).
From the first to the second trimester, urinary iodine excretion increased from 220.0 μg/L to 258.0 μg/L. Then in the third trimester, it decreased—to 195.0 μg/L. Postpartum, urinary iodine excretion was at 137.0 μg/L.
It was noted that, during pregnancy, maternal UIC correlated positively with serum TSH. No significant difference in UIC was seen between subjects with positive thyroid autoantibodies and those with negative antibodies.
The researchers concluded that iodine intake assessed by UIC in this population of pregnant and postpartum Japanese women is sufficient; it is not excessive, as per the World Health Organization criteria. The researchers also suggest that their results give additional information on UIC reference ranges during pregnancy in iodine-sufficient areas.