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 mug/l, indicating an iodine-sufficient population. 30.8% and 11.5% had urinary iodine levels 500 mug/l, respectively. When comparing mothers with urinary iodine levels /=150 mug/l, and >500 vs =500 mug/l, there were no differences in the risk of maternal increased or decreased TSH, hypothyroxinaemia or hyperthyroidism. Mothers with urinary iodine levels >500 mug/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