The implications of iodine and its supplementation during pregnancy in fetal brain development.

Iodine is an essential trace element for life. Its biological effects are a consequence of its incorporation to the thyroid hormones, which play a crucial role in fetal organogenesis, and in particular in brain development. This takes place during early gestation and involves delicate targeting throughout the central nervous system, including adequate neuronal growth, migration and myelinization at different sites, such as the cerebral cortex and neocortex, visual and auditory cortex, hippocampus and cerebellum. Pregnancy is characterized by an increased demand of thyroid hormones by the feto-placental unit in order to fulfill the necessary requirements of thyroid hormone action for normal fetal development. Up until week 20, the fetal thyroid is not fully active and therefore is completely dependent on the maternal thyroxine supply. Thus, the maternal thyroid has to adapt to this situation by producing about 1.5 fold more thyroxine. This requires that enzymatic gland machinery works normally as well as an adequate iodine intake, the principal substrate for thyroid hormone synthesis. Biological consequences of iodine related maternal hypothyroxinemia are currently very well known, by both experimental models and by clinical and epidemiological evidences. The associated disturbances parallel the degree of maternal thyroxine deficiency, ranging from increased neonatal morbi-mortality and severe mental dysfunction, to hyperactivity, attention disorders and a substantial decrease of IQ of an irreversible nature in the progeny of mothers suffering a deprivation of iodine during pregnancy. As a consequence, iodine deficiency is the leading preventable cause of mental impaired function in the world, affecting as many as 2 billion people (35.2% of the entire population). Prevention of fetal iodine deficiency - a problem of pandemic proportions- is feasible, provided that an iodine supply of 200-300 μg/day to the mother is ensured, before and throughout gestation as well as during the lactating period.