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

S. Gowachirapant

Research output: Thesisinternal PhD, WU

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 <14 weeks to term. Anthropometrics, maternal thyroid function, UIC and thyroid volume were measured at baseline, 2ndand 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] (P<0.001); 2) In the RCT, medianUIC increased significantly from baseline in both groups, and the increase was higher in the iodine group (p<0.001). At 30 weeks of gestation, only 2% of the women in the placebo group and 7% in the iodine group reported a median UIC >500 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<0.001) in early gestation (<14 weeks). Compared to normal weight women, the prevalence ratio (95% CI) of a low fT4 in overweight women was 3.64 (2.08–6.37) (P<0.01). In addition, secondary data analysis showed that overweight women had an 11-fold higher risk of delivering a large for gestational age infant compared to normal weight women, while women who had excessive GWG were 5.6 times more likely to deliver a macrosomic infant compared to women with normal GWG.

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.

Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
Supervisors/Advisors
  • Zimmermann, Michael, Promotor
  • Melse-Boonstra, Alida, Co-promotor
  • Winichagoon, P., Co-promotor, External person
Award date1 Jul 2014
Place of PublicationWageningen
Publisher
Print ISBNs9789462570290
Publication statusPublished - 2014

Keywords

  • iodine
  • nutritional state
  • trace element deficiencies
  • mineral supplements
  • pregnancy
  • thyroid function
  • infant development
  • thailand

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