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

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

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The Generation R Study Group, Erasmus Medical Center, The Netherlands; Department of Internal Medicine, Erasmus Medical Center, The Netherlands.



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.


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.


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.


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

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