Postpartum maternal iodine status and the relationship to neonatal thyroid function

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 mug/L) was found in 29 of the 50 subjects (58%). The median +/- standard deviation (SD) urine iodine was 46.8 +/- 28.5 mug/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 mug/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.