Context: Breastfed infants rely on adequate maternal dietary iodine intake.

Objective: To measure breast milk iodine and perchlorate, an inhibitor of iodide transport into the thyroid and potentially into breast milk, in Boston-area women.

Participants: 57 lactating healthy volunteers in the Boston area.

Measurements: Breast milk iodine and perchlorate concentrations, and urine iodine, perchlorate, and cotinine concentrations were measured. For comparison, iodine and perchlorate levels in infant formulae were also measured.

Results: Median breast milk iodine content in 57 samples was 155 µg/L (range 2.7 - 1968 µg/L). Median urine iodine was 114 µg/L (range 25 - 920 µg/L). Perchlorate was detectable in all 49 breast milk samples (range 1.3 - 411 µg/L), all 56 urine samples (range 0.37 - 127 µg/L), and all 17 infant formulae samples (range 0.22 - 4.1 µg/L) measured. Breast milk iodine content was significantly correlated with urinary iodine per gram creatinine and urinary cotinine, but was not significantly correlated with breast milk or urinary perchlorate.

Conclusions: Perchlorate exposure was not significantly correlated with breast milk iodine concentrations. Perchlorate was detectable in infant formula, but at lower levels than in breast milk. 47% of women sampled may have been providing breast milk with insufficient iodine to meet infants' requirements.