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

J Clin Endocrinol Metab. 2009 Nov;94(11):4444-7. doi: 10.1210/jc.2009-0632. Epub 2009 Oct 6.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Birth Weight
  • Breast Feeding / adverse effects
  • Female
  • Gestational Age
  • Humans
  • Hypothyroidism / epidemiology*
  • Infant, Newborn
  • Infant, Premature*
  • Iodine / analysis*
  • Korea / epidemiology
  • Milk, Human / chemistry*
  • Thyroid Function Tests
  • Thyroxine / blood


  • Iodine
  • Thyroxine