The brain in iodine deficiency.

Global descriptive, epidemiological studies have established the relation of iodine deficiency to endemic cretinism which, in its fully developed form, is characterized by mental deficiency, deaf mutism and spastic diplegia. However, a second less common variant--myxedematous or hypothyroid cretinism--is characterized by severe hypothyroidism with dwarfism. Mixed forms occur. It has been shown that both conditions can be prevented by correction of the iodine deficiency before pregnancy. Cretinism and development--now termed iodine deficiency disorders (IDD). A number of recently developed animal models establish the effect of severe iodine deficiency on brain development. These include the rat, the marmoset monkey and the sheep. These models are all characterized by the production of severe maternal and fetal hypothyroidism which is associated with effects on the maturation of the cerebral cortex and cerebellum. There was a reduced brain weight with a reduced number of cells as indicated by reduced DNA, a greater density of cells in the cerebral cortex and reduced cell acquisition in the cerebellum. Studies of the mechanisms involved have been carried out in the sheep. The findings reveal significant, though less severe, effects of fetal thyroidectomy (late gestation) and a significant effect of maternal thyroidectomy on brain development in mid-gestation. A combination of maternal and fetal thyroidectomy has similar but more severe effects than iodine deficiency. In the light of current knowledge of the embryology of the brain it is suggested that the critical time for the effect of iodine deficiency is the mid-trimester (14-18 weeks) when the neurons of the cerebral cortex and basal ganglia are formed and could be damaged by the effect of iodine deficiency on maternal thyroid function. There is now recent evidence indicating transfer of maternal thyroxine across the placental barrier early in pregnancy. In this way, neurological cretinism might be produced. Impaired fetal thyroid function would follow in the third trimester and augment the effect of reduced maternal thyroid function. Impaired fetal thyroid function alone could produce the hypothyroid form of cretinism. Further experimental studies, particularly into the postnatal period, are required to substantiate these suggestions. Apart from this, further study of the effects of iodine deficiency on brain development at the subcellular and cellular levels are likely to be most productive.