Subclinical hypothyroidism in Korean preterm infants associated with high levels of iodine in breast milk.

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 microg/liter), and one third of the infants had an iodine intake of more than 100 microg/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 microg/kg per day; P = 0.033), urinary iodine at the third week (913.2 +/- 1179.7 vs. 1651.3 +/- 1135.2 microg/liter; P = 0.051), and estimated daily iodine intake at the sixth week (32.8 +/- 35.5 vs. 92.1 +/- 51.2 microg/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.