Introduction:
Background. Secondary hypothyroidism is observed in children after brain damage. The aim of the study was evaluation of the mental development in preterm-born children during replacement therapy with l-thyroxin because of secondary hypothyroidism.

Material and Methods:
The motor and mental development of preterm newborns with secondary hypothyroidism treated with l-thyroxin since the second week of life were compared with the development of preterm newborns with secondary hypothyroidism treated since the fourth week, or later. Motor development was evaluated, and the mental development and IQ assessed by the Wechsler Intelligence Scale for Children in the seventh year of life.

Results:
Earlier achievement of the milestones of motor development, i.e. sitting, standing, and walking, was observed in children from the group who received early treatment with modest doses of l-thyroxin. In this group, all infants acquired the motor functions statistically significantly earlier in comparison to the infants from group with delayed treatment. In the seventh year of life, the IQs were significantly higher in group I treated since the second week of life, compared to group II.

Conclusions:
The early replacement therapy with l- thyroxin initiated in the second week of life may improve long-term mental development in children.