Context: The dietary iodine intake of lactating women has been reported to be high in Korea.

Objectives: The aim of this study was to assess iodine balance and to determine its relationship with thyroid function in preterm infants.

Design: Thyroid functions of preterm infants born at 34 wk gestation or less were evaluated in the first (n = 31) and third (n = 19) weeks. Mothers' breast milk (BM) and random urine samples of infants were taken on the same days for thyroid function tests.

Results: Iodine concentrations in BM were very high (198–8484 μg/liter), and one third of the infants had an iodine intake of more than 100 μg/kg per day at the third week after birth (excessive iodine intake group). At that time, the levels of TSH were positively correlated with urinary iodine (r = 0.622; P = 0.004). The frequencies of subclinical hypothyroidism were high in the excessive iodine intake group at the third and sixth weeks. The estimated daily iodine intake at the third week (51.2 ± 45.5 vs. 149.0 ± 103.8 μg/kg per day; P = 0.033), urinary iodine at the third week (913.2 ± 1179.7 vs. 1651.3 ± 1135.2 μg/liter; P = 0.051), and estimated daily iodine intake at the sixth week (32.8 ± 35.5 vs. 92.1 ± 51.2 μg/kg per day; P = 0.032) were significantly higher in infants with subclinical hypothyroidism than in controls.

Conclusions: Excessive iodine intake from BM contributed to subclinical hypothyroidism in these preterm Korean infants.