Postpartum Maternal Iodine Status and the Relationship to Neonatal Thyroid Function

Abstract

Iodine deficiency in the postpartum period has the potential to affect neonatal neuropsychointellectual development. We performed a cross-sectional study involving 50 postpartum women and their neonates, measuring maternal urine iodine, breast milk iodine, and neonatal thyroid stimulating hormone (TSH), and examining their interrelationships. Women were studied at a median (range) of 4 (3-9) days postpartum. Moderate to severe iodine deficiency (defined by urine iodine concentration < 50 microg/L) was found in 29 of the 50 subjects (58%). The median +/- standard deviation (SD) urine iodine was 46.8 +/- 28.5 microg/L and the mean urine iodine expressed in micrograms per gram of creatinine was 86.6 +/- 45.6. The median (range) breast milk iodine was 84.0 microg/L (25.0-234.0). Breast milk iodine was significantly correlated with urine iodine in micrograms per gram of creatinine (r = 0.52, p < 0.001) but not with urine iodine measured in micrograms per liter (r = 0.19, p = 0.2). Six percent of neonates had whole-blood TSH values of greater than 5 mIU/L. Neonatal TSH levels were positively correlated with higher breast milk iodine (r = 0.42, p = 0.003). There was no significant correlation between neonatal TSH levels and the mother's urine iodine content. There is a high prevalence of iodine deficiency in these lactating postpartum subjects. Urine iodine as micrograms per gram of creatinine is a good predictor of breast milk iodine content. In our study, higher breast milk iodine was correlated with a higher neonatal TSH. The impact of breast milk iodine content on neonatal TSH levels and neuropsychointellectual development needs further study.

Iodine Status of Pregnant Women and Their Progeny in the Minho Region of Portugal

“…The median iodine content in breast milk was lower than this in our study. Similarly to other published data, we also observed a positive correlation between the iodine content of breast milk and that of the nursing infant's urine (27)(28)(29)(30)(31)(32)(33)(34)(35). Considering that all of the women were breast-feeding on day 3 and 58% were doing so 3 months later, a high percentage of the infant population in the study area were probably not getting adequate iodine for thyroid hormone synthesis and storage (31)(32)(33)(34).…”

“…This is particularly relevant to the complications of ID because breast milk is usually the only source of iodine for newborns. Breast milk iodine is considered sufficient when values are in the range of 150-180 mg=L (4,11,14,17,18,(27)(28)(29)(30)(31)(32)(33)(34)(35). The median iodine content in breast milk was lower than this in our study.…”

“…Considering that all of the women were breast-feeding on day 3 and 58% were doing so 3 months later, a high percentage of the infant population in the study area were probably not getting adequate iodine for thyroid hormone synthesis and storage (31)(32)(33)(34). In fact, assuming a daily intake of 150 mL=kg (about 500 mL milk for the newborns and 750 mL for 3-month-old infants), these babies were getting about half of the recommended daily dose of iodine (1)(2)(3)31,35). Notably, infants taking formula milk at 3 months had a median UI significantly higher than breastfed infants (137 vs. 80 mg=L).…”

“…Other potential indices of ID, such as neonatal TSH, are not completely reliable due to many factors, including the timing of sample collection (1)(2)(3)(4)32,35,36). Although the present study showed that the newborns met the WHO criteria for desirable serum TSH concentrations (considered iodine sufficient if less than 3% of the newborns have TSH >5 mU=L), it is now well recognized that the regulation of TSH secretion in the newborn does not follow the same feedback mechanisms observed when the thyroid-pituitary axis is fully developed (6).…”

Gestational changes of thyroid function and urinary iodine in thyroid antibody-negative Japanese women

“…Fourthly, the FFQ retrospectively evaluates daily iodine intake over a period of time, whereas the UIC reflects iodine intake over the recent few days rather than usual intake. Lastly, it has been reported that the level of iodine intake measured by the FFQ tends to be overestimated and the reason is eral studies including our previous study [24], those in the mildly iodine-deficient areas from Australia [57][58], and in the pregnant women with extremely high UI excretion (1.2mg/L) in China [59], whose UIC positively correlated with neonatal FT 4 concentration. The exact reason for the discrepancies is not clear and further large epidemiological studies are necessary in different iodine statuses.…”