Women with high early pregnancy urinary iodine levels have an increased risk of hyperthyroid newborns: the population-based Generation R Study

Clin Endocrinol (Oxf). 2014 Apr;80(4):598-606. doi: 10.1111/cen.12321. Epub 2013 Oct 9.


Objective: Iodine deficiency during pregnancy results in thyroid dysfunction and has been associated with adverse obstetric and foetal effects, leading to worldwide salt iodization programmes. As nowadays 69% of the world's population lives in iodine-sufficient regions, we investigated the effects of variation in iodine status on maternal and foetal thyroid (dys)function in an iodine-sufficient population.

Design, participants and measurements: Urinary iodine, serum TSH, free T4 (FT4) and TPO-antibody levels were determined in early pregnancy (13·3 (1·9) week; mean (SD)) in 1098 women from the population-based Generation R Study. Newborn cord serum TSH and FT4 levels were determined at birth.

Results: The median urinary iodine level was 222·5 μg/l, indicating an iodine-sufficient population. 30·8% and 11·5% had urinary iodine levels <150 and >500 μg/l, respectively. When comparing mothers with urinary iodine levels <150 vs ≥150 μg/l, and >500 vs ≤500 μg/l, there were no differences in the risk of maternal increased or decreased TSH, hypothyroxinaemia or hyperthyroidism. Mothers with urinary iodine levels >500 μg/l had a higher risk of a newborn with decreased cord TSH levels (5·6 ± 1·4 (mean ± SE) vs 2·1 ± 0·5%, P = 0·04), as well as a higher risk of a hyperthyroid newborn (3·1 ± 0·9 vs 0·6 ± 0·3%, P = 0·02). These mothers had newborns with higher cord FT4 levels (21·7 ± 0·3 vs 21·0 ± 0·1 pm, P = 0·04). Maternal urinary iodine levels <150 μg/l were not associated with newborn thyroid dysfunction.

Conclusions: In an iodine-sufficient population, higher maternal urinary iodine levels are associated with an increased risk of a hyperthyroid newborn.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Female
  • Fetal Blood
  • Humans
  • Hyperthyroidism / blood*
  • Infant, Newborn
  • Infant, Newborn, Diseases / blood*
  • Iodides
  • Iodine / urine*
  • Mothers
  • Pregnancy
  • Thyrotropin / blood
  • Thyroxine / blood


  • Iodides
  • Thyrotropin
  • Iodine
  • Thyroxine