Variation of iodine concentration in breast milk and urine in exclusively breastfeeding women and their infants during the first 24 wk after childbirth

•Iodine nutrition of exclusively breastfeeding women and their infants during the first 24 wk after childbirth were sufficient.•No difference but a strong correlation was found between iodine concentration in bilateral breast milk.•Positive and significant correlations among urinary iodine concentration in infants and their lactating mothers, and breast milk iodine concentration were found.•Breast milk iodine concentration declined from week 1 to week 8 postpartum.•Urinary iodine concentration in infants and their lactating mothers dropped from week 1 to week 4 postpartum then stabilized.