Thyroid hormones play a crucial role in child development. In infants, too little thyroid hormone (hypothyroidism) is deleterious, especially for brain development, and untreated hypothyroidism in infancy leads to irreversible intellectual disability. To avoid this, newborn screening programs have been implemented to diagnose hypothyroidism early. Congenital hypothyroidism is most often due to a thyroid that has not developed in the correct location (thyroid ectopy) and does not produce enough hormones. In the most severe cases, there is complete absence of the thyroid (athyreosis). The reason why the thyroid gland does not attain its proper place in the neck is largely unknown. Therefore, it is important to better understand this disease, to diagnose it early and to optimize treatment. Early detection by taking a few drops of blood from the heel of the newborn, even before discharge from the hospital, allows most affected children to attain their full intellectual potential, with one tablet of thyroid hormone daily. However, developmental delay may still occur in some severely affected children. A loss of 10 points of intelligence quotient has a major impact for the affected individual and for society. We estimate a global economic benefit of 1 to 1.3 Million $ per year (in Canada) for each IQ point gained in patients suffering from severe congenital hypothyroidism, i.e. thyroid ectopy and athyreosis. However, nearly 40 years after implementation of routine screening, the cause of congenital hypothyroidism remains largely unknown. The goal of our research is to discover the mechanisms underlying abnormal thyroid gland development and possible markers predicting loss of intellectual potential so as to provide prompt remedial help to children needing it.