A survey of the iodide concentration of human milk.

The iodide concentration of human milk from mothers living in the Wellington area and of six prepared infant formulae was measured. A total of 93 human milk samples were assayed, 14 from mothers of infants delivered at less than 35 weeks gestation and collected within 30 days of delivery (group A), 57 samples from mothers of infants delivered at 35 or more weeks gestation and collected within 60 days of delivery (group B) and 22 samples from mothers with infants aged greater than 60 days (group C). There was no significant (p greater than 0.05) difference between the means (SE) of groups A 1.14 mumol/L (0.20) and B 0.98 mumol/L (0.07). The mean for group C 0.39 mumol/L (0.03) was significantly less (p less than 0.05) than the means for both groups A and B. For groups A, B and C combined, there was a correlation (r = 0.44, p less than 0.01) between the milk iodide concentration and the iodide/creatinine ratio of a maternal urine specimen obtained immediately before or after the milk specimen. Formula milk preparations based on cow's milk contained higher concentrations of iodide than human milk.