There are presented the results from a cognition follow-up study that took 15 years, in children diagnosed as having congenital hypothyroidismin in the prevention program for mental retardation. The objectives are:

• To know the typical cognitive pattern in critical stages of the development of children with CH precociously treated,
• To identify the effect from the factors of the severity of the disease and the treatment on the neurocognitive, clinical and hormonal development of the cases from childhood until adolescence,
• To develop, whenever possible, stimulation and rehabilitation techniques of cognitive deficit processes,
• To contribute understanding the action of thyroid hormones on brain development.

The main scientific contributions from these researches are:

• Benefit demonstration of high initial dose without noxious effect on the attention,
• Identification of the T4 levels before the treatment and the average T4 during the first year of life as reliable forecasters of cognitive long term development,
• Differential effect of the duration of fetal hypothyroidism and of the initial severity on the cognition,
• Demonstrating that the deficits in the fine psycho motor skills are relatively independent from the severity of the disease and of the treatment efficacy. A hypothesis is formulated about the double genetic effect on the cognition of these children.