Neonatal thyroid function born to mothers living with long-term excessive iodine intake from drinking water.

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.