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 mu g/liter), and one third of the infants had an iodine intake of more than 100 mu g/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 mu g/kg per day; P = 0.033), urinary iodine at the third week (913.2 +/- 1179.7 vs. 1651.3 +/- 1135.2 mu g/liter; P = 0.051), and estimated daily iodine intake at the sixth week (32.8 +/- 35.5 vs. 92.1 +/- 51.2 mu g/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. (J Clin Endocrinol Metab 94: 4444- 4447, 2009)