Effects of iodine supplementation in mild-to-moderately iodine-deficient pregnant women on thyroid function, pregnancy outcomes and newborn development in Thailand

Abstract

Background: Iodine deficiency (ID) during pregnancy has been recognized as a major cause of hypothyroidism and adverse health consequences in both mothers and children. Although urinary iodine concentration (UIC) in school-aged children is recommended as an indicator to assess ID in the general population, it may not be a good surrogate for directly assessing iodine status in pregnant women. Iodine supplementation of mildly iodine-deficient pregnant women has been recommended worldwide; however, long-term benefit and safety of iodine supplementation in this group is uncertain. Finally, pre-pregnancy body mass index (BMI) and gestational weight gain (GWG) may negatively affect thyroid function and pregnancy outcomes. Objectives: 1) to measure UIC in pairs of pregnant women and their school-aged children living in the same household; 2) to investigate the effects of iodine supplementation on maternal thyroid function, pregnancy and birth outcomes, and newborn development; 3) to evaluate the association between pre-pregnancy BMI and GWG with thyroid function and pregnancy outcomes. Methods: 1) In a cross-sectional pilot study, UIC was measured in spot urine samples from pairs (n=302) of healthy pregnant mothers and their school-aged children in Bangkok; 2) Arandomized controlled trial was conducted with 200 µg iodine per day or placebo given to mildly ID pregnant Thai women from ndand 3rdtrimester, at delivery and 6-week postpartum. Birth outcomes were collected from hospital records. Neonatal thyroid function, UIC and thyroid volume were measured at delivery and 6 weeks after birth. The Neonatal Behavioral Assessment Scales (NBAS) was used to assess newborn development. Results: 1) In the pilot study, median UIC in the pregnant women was 108 (11–558) µg/L and was lower than UIC in their school-aged children [200 (25–835) µg/L] (P500 µg/L. Maternal thyroid function, thyroid volume and the prevalence of all thyroid dysfunction subtypes did not differ significantly between treatment groups during the study (p>0.05). At 6-week postpartum, the prevalence of postpartum thyroiditis (hyperthyroidism) was significantly lower in the iodine group (3%) as compared to the placebo group (9%) (OR: 95%CI, 0.17: 0.04-0.70). There were no significant differences between newborn groups in thyroid function, thyroid volume, birth characteristics, UIC and NBAS score (p>0.05); 3) Pre-pregnancy BMI was a negative predictor of free thyroxine (fT4) (β=-0.20, P Conclusion: 1) UIC in school-aged children should not be used as a surrogate for monitoring iodine status in pregnancy; 2) iodine supplementation (200 µg/d) in mildly iodine-deficient pregnant Thai women was effective in increasing iodine intakes into the adequate range but had no benefit on antenatal maternal thyroid function or newborn outcomes out to 6 weeks; however, it significantly reduced the risk of maternal postpartum thyroid dysfunction; 3) excess maternal body weight both before and during pregnancy may have adverse impacts on maternal thyroid function as well as birth weight. Therefore, maintaining normal body weight before and throughout pregnancy should be recommended.</p