Iodine Status of Pregnant and Postpartum Japanese Women: Effect of Iodine Intake on Maternal and Neonatal Thyroid Function in an Iodine-Sufficient Area

Background: Iodine deficiency in pregnant and lactating women results in serious damage to their fetuses, newborns, and weaning infants. The effect of dietary iodine intake on maternal and infantile thyroid function has not been well studied in iodine-sufficient areas, and there are few data on appropriate gestational age-specific reference ranges for urinary iodine excretion during pregnancy and lactation. Objectives: The aim of the study was to characterize the gestational change of urinary iodine excretion in Japanese women and to assess the effects of iodine status on thyroid function in mother and infant. Methods: A total of 934 Japanese women and their 722 new born infants were enrolled in the study. Iodine and creatinine concentrations were determined in spot urine samples in the three trimesters of pregnancy and the postpartum period at 34.0 d after delivery. Serum thyroperoxidase antibody and thyroglobulin antibody, TSH, and free T-4 were measured in each trimester, and neonatal TSH was measured on postnatal d 4. Results: The overall median urinary iodine concentration (UIC) during pregnancy was 219.0 mu g/liter, higher than that in postpartum women (135.0 mu g/liter). The prevalence of pregnant women with low UIC less than 100 mu g/liter or high UIC greater than 500 mu g/liter was 16.1 and 22.2%, respectively. Urinary iodine excretion increased from 220.0 mu g/liter in the first trimester to 258.0 mu g/liter in the second trimester, decreased to 195.0 mu g/liter in the third trimester, and then remained at 137.0 mu g/liter postpartum. The maternal UIC correlated positively with serum TSH during pregnancy. There was no significant difference in UIC between subjects with positive thyroid autoantibodies and those with negative antibodies. Conclusions: Iodine intake assessed by UIC in Japanese pregnant women is regarded as sufficient and not excessive according to World Health Organization criteria. Although the data are local, our results provide additional information on the reference range for UIC throughout gestation in iodine-sufficient areas. (J Clin Endocrinol Metab 96: 3846-3854, 2011)