Thyroxin and triiodothyronine (T4 and T3) are crucial for the satisfactory morphological and functional development of the central nervous system during the intrauterine and postnatal period. Contemporary embryological knowledge stratifies the development of cerebral structures into time zones and associations and thus it is known during which week of gestation a certain section is formed. The association between the availability of T3 in the nuclei of neurones and glial cells at the correct time and in the correct amount and implementation of the sequence of development of essential brain structures has been already proved. Mechanisms are also known which regulate the transport of maternal T4 to the cells of the embryo and foetus and the subsequent sophisticated part played by deiodases which regulate very sensitively the local production of T3. The prerequisite of these objectives is however proper function of the maternal thyroid in particular during the first half of pregnancy. The iodine requirement during pregnancy is high. After delivery when the development of the CNS proceeds and the child must ensure his own formation of T4 and T3, the need of a permanent iodine supply persists. The adaptational power of the sodium-iodide symporter (NIS) in thyrocytes of lactating women ismore than doubled. Care of proper thyroid function is necessary not only in women of fertile age, however, the sequealae of hypothyroxinaemia are most serious in the latter and are irreparable in the child.

        Key words: stages of CNS development, role of T3, transplacental T4 transport, deiodases, hormonogenesis of the foetus, recommendations for practice