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

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 center dot 3 (1 center dot 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 center dot 5 mu g/l, indicating an iodine-sufficient population. 30 center dot 8% and 11 center dot 5% had urinary iodine levels <150 and >500 mu g/l, respectively. When comparing mothers with urinary iodine levels <150 vs >= 150 mu g/l, and >500 vs <= 500 mu g/l, there were no differences in the risk of maternal increased or decreased TSH, hypothyroxinaemia or hyperthyroidism. Mothers with urinary iodine levels >500 mu g/l had a higher risk of a newborn with decreased cord TSH levels (5 center dot 6 +/- 1 center dot 4 (mean +/- SE) vs 2 center dot 1 +/- 0 center dot 5%, P=0 center dot 04), as well as a higher risk of a hyperthyroid newborn (3 center dot 1 +/- 0 center dot 9 vs 0 center dot 6 +/- 0 center dot 3%, P=0 center dot 02). These mothers had newborns with higher cord FT4 levels (21 center dot 7 +/- 0 center dot 3 vs 21 center dot 0 +/- 0 center dot 1 pm, P=0 center dot 04). Maternal urinary iodine levels <150g/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.