Breast-milk iodine concentration declines over the first 6 mo postpartum in iodine-deficient women.

Randomized, double-blind, placebo-controlled trial in healthy lactating women in Dunedin, New Zealand. Eligible participants had a singleton birth, no history of thyroid disease, were not taking an iodine-containing supplement, and intended to breastfeed for 24 weeks; the active arms included 27 women assigned to 75 μg/day and 26 assigned to 150 μg/day, with allocation ratio 2:1:1 across placebo, 75 μg/day, and 150 μg/day.

Healthy lactating women received a daily iodine supplement for 24 weeks postpartum: 75 μg/day or 150 μg/day, given as 1 tablet/day of potassium iodate, compared with placebo. Treatment began a mean of 5.6 ± 4.2 days postpartum; reported mean compliance was 86 ± 17%.

Iodine supplementation modestly improved breast-milk and maternal iodine markers, but the doses tested did not achieve adequate iodine status in mothers or infants. Between weeks 1 and 24, breast-milk iodine concentration fell by 40% in the placebo group (P < 0.001), while it was higher with 75 μg/day versus placebo (1.31 times higher; 95% CI: 1.02, 1.67; P = 0.030) and with 150 μg/day versus placebo (1.69 times higher; 95% CI: 1.32, 2.16; P < 0.001). Maternal urinary iodine concentration also increased with 75 μg/day (2.12 times; 95% CI: 1.61, 2.78; P < 0.001) and 150 μg/day (2.35 times; 95% CI: 1.78, 3.09; P < 0.001) versus placebo, but maternal fT4 and TSH did not differ between groups. Infant urinary iodine remained low overall; the 150 μg/day group was higher than placebo, but 75 μg/day was not clearly different, and the authors concluded that higher iodine doses would be needed to meet infant requirements.

The intervention was tested in small active arms over a relatively short 24-week postpartum period, limiting precision and long-term inference. No arm-specific age data, ethnic data, or adverse event results were reported, and generalizability is limited to healthy iodine-deficient lactating women in New Zealand. The tested doses improved biomarkers but were still insufficient to normalize iodine status, especially for infants.