Breast milk contains perchlorate which decreases thyroid hormone production in infants.
Perchlorate is a widespread contaminant found in drinking water, foods, and in some women's breast milk. Large amounts block iodine uptake by the thyroid...potentially decreasing thyroid hormone production. Thyroid hormones are critical for normal growth and development of the central nervous system of fetuses and infants.
Concerns with perchlorate toxicities led to the development of guidelines for safe exposure to perchlorate. The EPA and National Science Academy consider a daily exposure of 0.7 mg/kg/day as not likely to cause harm.
A recent study reports that levels of perchlorate in some women's breast milk can expose babies to 3 times or higher this amount.