Iodine Concentrations in Milk and in Urine During Breastfeeding Are Differently Affected by Maternal Fluid Intake

Background: Breastfed infants are dependent on iodine transport into breast milk for production of thyroid hormones. Thyroid hormones are important regulators of brain development. It has been considered whether breast milk iodine concentration (MIC) could be predicted by maternal urinary iodine concentration (UIC), but reports on correlations have been inconsistent. We used urinary creatinine concentration as a proxy for maternal fluid intake and speculated if this might differently influence UIC and MIC. Methods: We examined 127 breastfeeding women after the introduction of the mandatory iodine fortification of salt in Denmark. Maternal spot urine and a breast milk sample were obtained at a median of 31 days after delivery (interquartile range: 25–42 days), and the women were asked about intake of iodine containing supplements postpartum. Results: Median UIC was 72 μg/L (46–107 μg/L) and higher in iodine-supplemented mothers (47.2% of participants); 83 μg/L (63–127 μg/L) versus 65 μg/L (40–91 μg/L), p=0.004. Median MIC was 83 μg/L (61–125 μg/L) and also higher in iodine-supplemented mothers; 112 μg/L (80–154 μg/L) versus 72 μg/L (47–87 μg/L), p<0.001. There was a weak correlation between UIC and MIC (r=0.28, p=0.015). A strong correlation was present between UIC and urinary creatinine concentration (r=0.76, p<0.001), whereas urinary creatinine concentration was not correlated to MIC (r=−0.049, p=0.58). When UIC and urinary creatinine were used to estimate 24-h urinary iodine excretion, the correlation between this estimate and breast milk iodine excretion was stronger (r=0.48, p<0.001). Conclusions: Intake of an iodine supplement should be recommended in Danish breastfeeding women. Our results indicate that UIC, but not MIC, depends on maternal fluid intake and that maternal estimated 24-h iodine excretion may be a better indicator of iodine supply to the breastfed infant than UIC.