Abstract and Introduction
Objective The effects of long-term excessive maternal iodine intake on neonatal thyroid function are less known. This study aimed to assess the effects of maternal excessive iodine intake from drinking water on thyroid functions of both mothers and their neonates.
Design and Methods This observational study was performed in high iodine (HI) areas and adequate iodine (AI) intake areas, including 384 healthy pregnant women in late gestation (mean week 39·3 ± 1·6 weeks) and their newborns. Blood and urine samples were obtained from pregnant women, while cord blood samples were obtained from neonates. Urinary iodine concentration (UIC) and thyroid function were evaluated.
Results The median maternal UIC was 1241 and 217 μg/l in HI and AI areas, respectively (P < 0·001). The concentrations of serum thyroid-stimulating hormone (TSH), free triiodothyronine (FT3) and free thyroxine (FT4) in neonates in HI areas were 7·33 mIU/l (range 5·47, 11·06 mIU/l), 2·93 ± 0·59 and 15·03 ± 1·92 pmol/l, respectively, while that were 4·71 mIU/l (range 3·96, 6·04 mIU/l), 2·31 ± 0·28 and 16·50 ± 1·35 pmol/l in AI neonates (P < 0·05). Similar changes were also observed in neonates in HI areas when excluding the effect of maternal thyroid autoimmunity. Cord blood TSH concentration (r = 0·31, P = 0·001) and FT3 concentration (r = 0·43, P = 0·001) were positively correlated with maternal UIC. Cord blood FT4 concentration was negatively correlated with maternal UIC (r = −0·25, P = 0·001). Mothers living in HI areas (β = 0·296, 95% CI: 0·163, 0·255) and with subclinical hypothyroidism (β = 0·360, 95% CI: 0·034, 0·175) contributed to elevated cord blood TSH concentration in neonates, while male neonates were more likely to present with higher TSH concentration compared with female infants (β = −0·760, 95% CI: −0·119, −0·033).
Conclusions Excessive iodine intake during pregnancy was associated with an increased rate of hyperthyrotropinaemia in neonates and their mothers, especially in male neonates.
Optimal iodine nutritional status during pregnancy is essential for maintaining both proper thyroid function in mothers and normal development of the brain of progeny during foetal and early post-natal life. As neonatal thyroid function has not been established in the early trimester of pregnancy, so the growth and development of the foetus mainly rely on maternal thyroid hormones. Because iodine is essential for the synthesis of thyroid hormones, iodine deficiency in the mother may lead to insufficient thyroid hormone synthesis in mothers and foetus, resulting in the impairment of the neuropsychological development in infants. Therefore, the iodine requirement of the mother during pregnancy and lactation is increased. The daily iodine intake during pregnancy recommended by World Health Organization/United Nations Children's Fund/The International Council for the Control of Iodine Deficiency Disorders (WHO/UNICEF/ICCIDD) was 200 μg in 1996 and 250 μg in 2007. In recent years, there have been increasing concerns about pregnant and lactating women and their weaning infants who do not receive sufficient iodine. The problem even exists in those countries that have sufficient iodine supplies for several decades. The American Thyroid Association and The Endocrine Society recommend that pregnant and lactating women in North America where mild iodine deficiency is common should ingest an iodine supplementation containing 150 μg iodine per day while the total iodine intake not exceeding 500 μg/day.
On the other hand, it is well known that excess iodine has an inhibitory effect on the thyroid gland. Previous studies have suggested that foetal thyroid is more susceptible to this inhibitory effect. While iodine supplementation could improve iodine nutrition and normalize the impaired thyroid function in iodine deficient mothers, it could also inhibit foetal thyroid function when the iodine level is excessive. For instance, studies conducted in the iodine-sufficient areas in Japan observed a transient elevation of thyroid-stimulating hormone (TSH) in some neonates born to the Japanese mothers who were supposed to have consumed the iodine-rich food during their pregnancy or lactation. In addition, iodine contamination has been proved to induce a transient elevation of TSH concentration in neonates, for example, the use of iodine-containing disinfectants in mothers. It has been reported that neonatal goitre, hyperthyroid and hypothyroidism could be induced by excessive maternal iodine ingestion. Iodine excess also affects maternal thyroid function. Our previous study showed that subclinical hypothyroidism was the main pattern of thyroid dysfunctions in pregnant women with an excessive iodine intake from drinking water.
However, little is known about the effects of excessive maternal iodine intake on neonatal thyroid function in a population that has been chronically exposed to high doses of iodine mainly from drinking water. Thus, this study aimed to investigate the effects of excessive maternal iodine intake on thyroid function in neonates during pregnancy.