Central nervous system thyroid hormones effects

In addition to the obvious deactivating role of deiodinases, there has been recent evidence that a relationship exists between regulation of deiodination of thyroid hormones in target cells and the intracellular effects of T4 and T3 on pituitary and hypothalamus function. In the rat pituitary, and probably the human, type-II deiodinase-catalyzed conversion of T4 to T3 is a prerequisite for inhibition of TRH release. rT3, produced from T4 by type-III deiodinase, is a potent inhibitor of type-II deiodinase. In a postulated regulatory circuit, rT3 formed from T4 by type-III deiodinase in surrounding CNS (Central Nervous System) tissue enters the pituitary and inhibits type-II enzyme. The resulting decrease in T3 concentration, in turn, causes an increase in TSH secretion49. 

Q3 The thyroid hormones thyroxine and triiodothyronine have many metabolic effects. In adults they increase metabolic rate, oxygen and calorie consumption, stimulate carbohydrate metabolism and turnover of protein, deplete fat stores and increase catabolism of free fatty acids. Thyroid hormones stimulate heart rate and force and increase pulmonary ventilation, gastrointestinal motility and central nervous system (CNS) activity. Actions on the heart can result in an increased incidence of dysrhythmias. Thyroid hormones are critical for the normal growth and development of the infant, particularly in respect of skeletal growth and maturation of the CNS. 

The organization of the endocrine system can best be described in relation to the central nervous system. Three levels of endocrine tissues can be distinguished on the basis of their association with the central nervous system (Figure 30-12). The first level consists of those that are (or were) derived from nerve cells these include the hypothalamus, adrenal medulla, thyroid C-cell, and gastrointestinal enterochromaffin cells. The hypothalamus and adrenal medulla still retain their neural connections and can therefore be regarded as endocrine extensions of the nervous system. The C-cell and the gut cells, however, are APUD cells and lack neural connections. These four tissues produce hormonal peptides or amines having, like neurotransmitters, rapid-onset, short-term effects. 

Effects of Thyroid Hormones on the Human Central Nervous System during Fetal and Postnatal Life... 

This report has briefly summarized what we now know about thyroid hormone transport to the central nervous system. The data are still sketchy and much remains to be done. Obviously the brain is a complex organ and major differences in thyroid hormone transport and metabolism are to be expected in its constituent parts, so study of different brain regions as well as different cell types is required. We also need to distinguish between findings in the mature brain and those during fetal and postnatal development. Thyroid hormones play a very different role in these stages of the organism, and possible variations in hormone delivery to cells may contribute to these differences. Finally, in the malnutrition that often accompanies iodine deficiency, we need to ask whether PA synthesis in the choroid plexus is compromised, as it is in the liver. If so, important effects in thyroid hormone delivery to the brain may be expected. 

J. Legrand. Effects of thyroid hormones on central nervous system... 

Legrand, J., 1984, Effects of thyroid hormones on central nervous system development, in "Neurobehavioral Teratology , J. Yanai, ed., Elsevier, Amsterdam, p 331. 

Earlier studies on hypothyroid animals were mainly concentrated on the postnatal period, and little is known about the effects of hormonal rehabilitation after weaning. In addition, major advances in our knowledge of cell-specific markers in the central nervous system,and about tissue culture methods,24 now makes it possible to ask penetrating questions about the direct effect of the thyroid hormone and its possible interaction with centrally produced humoural factors. 

Chlorobenzenes are persistent and bioacciunulative chemieals that have been used as solvents and biocides, in the manufacture of dyes and as chemical intermediaries. The effects of exposure depend on the type of ehlorobenzene however, they eommonly affeet the liver, thyroid and central nervous system. Hexachlorobenzene (HCB), the most toxic and persistent chemieal of this group, is also a hormone disrupter. 

The effects of thyroid hormones on the nervous system can be both central and peripheral. Cerebral, cranial nerve, peripheral nerve, neuromuscular junction and muscles may be affected by thyroid status. Pathological changes are also known to occur within the eye. 

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