Thyroid hormones neural development

Early effects of T3 on neuroblast division and differentiation occur before the neural apparatus for processing thyroid hormones has developed. The period of thyroid hormone-dependent nerve cell specialization occurs later, during the so-called critical period, coinciding with the elaboration of a T3-processing neural system. In the differentiated brain there is now evidence compatible with a role for T3 in maintaining neural plasticity and intercellular communication through a transsynaptic mechanism. 

Table 49.3 Effect of thyroid hormone on neural development...

The effect of the thyroid hormone on brain development has been reviewed by Bernal (2002), who has indicated the many neurobiological actions of the hormone on developing neural tissue, in addition to documenting some of the genes affected by T3 (Table 49.4). 

The two most recent NHANES surveys showed that for women of childbearing age and pregnant women, the median UI excretion levels were adequate (127 and I4lpg/1, respectively, in 1988-1994 132 and 173 pg/1, respectively, in 2001-2002) (Table 115.3) (Caldwell et al., 2005). Thyroid hormone, requiring adequate iodine intake, is critical for neural development in utero and in early life. Although cretinism due to iodine deficiency is not a problem in the United States, subtle developmental delays could result from mild maternal iodine deficiency. 

Although cretinism was already described by the turn of the century, the involvement of thyroid deficiency in this malady was not reported until the 1930 s (Kerley, 1936). Soon thereafter, animal studies were carried out which made clear the necessity of thyroid hormones for general somatic and neural development during the neonatal and early postnatal period in the rat, which is roughly equivalent to the first year of life in the human (Salmon, 1936 Scow and Simpson, 1945 Eayrs and Taylor, 1951 Eayrs and Horn, 1955 Hamburg and Vicari, 1957 Eayrs, 1961). 

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