Objectives: Some European countries still suffer from Mild Iodine Deficiency (MID). MID during pregnancy may impair intellectual, psychosocial and psychomotor development of children and may lead to Attention deficit and hyperactivity disorder (ADHD). Neonatal TSH concentration is a good indicator of iodine deficiency in late pregnancy. This study aims to investigate the association between neonatal TSH level and intellectual, psychomotor and psychosocial development of 4-5 year old children. It is hypothesized that elevation of TSH at birth is associated with impaired intellectual and psychomotor development and with behavioural problems at 4-5 years.Methods: The study will include 380 Belgian preschool children with a TSH concentration between 0 and 15 mU/L at screening. For each sex and TSH-interval (0-1 mU/L, 1-2 mU/L, 2-3 mU/L, 3-4 mU/L, 4-5 mU/L, 5-6 mU/L, 6-7 mU/L, 7-8 mU/L, 8-9 mU/L, 9-15 mU/L) 19 newborns will be randomly selected after excluding infants with congenital hypothyroidism, low birth weight and premature infants. Neonatal TSH was measured in dried blood spots collected by heel stick 3 to 5 days after birth using the Autodelphia method. Cognitive abilities and psychomotor development will be assessed using respectively the Wechsler Preschool and Primary Scale of Intelligence-III and the motor scale of the McCarthy Scales of Children's Abilities. Psychosocial development will be measured using the Child Behaviour Check List for ages 1½-5 years. In addition the parents will complete a general questionnaire in order to account for confounding factors.Results: The results of the study will be available by September 2013.Conclusions: The study might have implications on the use of neonatal TSH screening results for monitoring iodine intake among the population and might require definition of new TSH cut-offs to be used by neonatal screening centres in order to recall certain neonates.