Thyroid hormone control

Fig. 4. Action of thyroid hormones on cyclic AMP production and degradation in the adipocyte. The response of the adipocyte to different lipolytic hormones (/3 catecholamines, ACTH and glucagon) is under thyroid hormone control both at the level of the receptor-adenylate cyclase complex and at the level of the phosphodiesterase. T, also regulates the expression of several key lipogenic enzymes.

Because most of the energy produced by cellular respiration eventually appears as heat, an increase in cellular respiration necessarily leads to an increase in heat production (i.e., to a thermogenic or calorigenic effect). Thus, to the degree that thyroid hormones control BMR, they also control thermogenesis (17). 

The thyroid hormones control metabolism. They increase cardiac output and the excitability of the nervous system. In children, the maturing of the cerebral cortex, of the skeleton, the musculature and the genitalia, is stimulated. Both hormones are active only in their protein-imbound state. Free thyroxine and 3,3 ,5-triiodothyronine bind to receptors in the ceU nucleus and in the mitochondria, where they activate protein synthesis and the production of adenosine triphosphate. 

Since the brain contains receptors and is a developmental and homeostatic target for thyroid hormones, a mechanism must exist which discriminates between neural and peripheral tissue-specific functions under thyroid hormone control. We describe here the properties of a thyroid hormone receptor predominantly expressed in the rat brain. We also present in situ hybridization histochemistry data outlining the brain subregions containing this receptor mRNA. Its characterization implies a multiple receptor system which may help to explain some of the tissue-specific physiological effects of thyroid hormones. 

S.A. Stein, Thyroid hormone control of gene espression in Sprague-... 

There are a number of possibilities that could account for the interaction between thyroid hormone and NGF. For example, treatments with insulin and insulin-like growth factor II have been found to increase specific and saturable NGF binding sites in cultured human neuroblastoma cells.It is possible that analogous mechanisms between thyroid hormone and NGF may operate in subcortical cholinergic cells. Another possibility could be that if thyroid hormone controls the formation of ChAT or of proteins essential for cholinergic cell maturation at a pretranslational level S and NGF regulates the de novo synthesis of these crucial proteins at transcriptional level,30 then a combination of these effects could synergistically potentiate ChAT activity. 

Marsh-Armstrong N., Cai L, Brown DD (2004) Thyroid hormone controls the development of connections between the spinal cord and limbs during Xenopus laevis metamorphosis. Proc Nad Acad Sci USA 101 165-170. 

Metabolic Functions. The functions of the thyroid hormones and thus of iodine are control of energy transductions (121). These hormones increase oxygen consumption and basal metaboHc rate by accelerating reactions in nearly all cells of the body. A part of this effect is attributed to increase in activity of many enzymes. Additionally, protein synthesis is affected by the thyroid hormones (121,122). 

Fig. 1. Mechanisms controlling free thyroid-hormone levels.

Hyperthyroidism, that is, the overproduction of thyroid hormones, is usually treated by surgical removal of the thyroid gland. Before such a procedure is undertaken, the hyperthyroidism is usually first brought under control by treatment with so-called antithyroid agents. 

Cellular functions are controlled by extracellular signals such as hormones, neurotransmitters, odorants, light and other chemical or physical stimuli. Only a few of these signal molecules, e.g., the highly lipid-soluble steroids or thyroid hormones, can diffiise across the... 

The formation of FMN and FAD is ATP-dependent and takes place predominantly in liver, kidney, and heart. It is controlled by thyroid hormones [2]. 

Most of the physiologic activity of thyroid hormones is from the actions of T3. T4 can be thought of primarily as a prohormone. Eighty percent of needed T3 is derived from the conversion of T4 to T3 in peripheral tissue under the influence of tissue deiodinases. These deiodinases allow end organs to produce the amount of T3 needed to control local metabolic functions. These enzymes also catabolize T3 and T4 to biologically inactive metabolites. Thyroid hormones bind to intracellular receptors and regulate the transcription of various genes. 

Hormonal actions occur during sensitive periods in development, in adult life during natural endocrine cycles and in response to experience as well as during the aging process (see Ch. 30). As a result of their fundamental actions on cellular processes and genomic activity and of the control of their secretion by environmental signals, steroid and thyroid hormone actions on the brain provide unique insights into the plasticity of the brain and behavior (see also Ch. 50). 

Forrest, D., Reh, T. A. and Rusch, A. Neurodevelopmental control by thyroid hormone receptors. Curr. Opin. Neurobiol. 12 49-56,2002. 

Reports of the effects of Li+ upon the thyroid gland and its associated hormones are the most abundant of those concerned with the endocrine system. Li+ inhibits thyroid hormone release, leading to reduced levels of circulating hormone, in both psychiatric patients and healthy controls [178]. In consequence of this, a negative feedback mechanism increases the production of pituitary TSH. Li+ also causes an increase in hypothalamic thyroid-releasing hormone (TRH), probably by inhibiting its re-... 

Thyroid hormone production is regulated by TSH secreted by the anterior pituitary, which in turn is under negative feedback control by the circulating level of free thyroid hormone and the positive influence of hypothalamic thyrotropin-releasing hormone. Thyroid hormone production is also regulated by extrathyroidal deiodination of T4 to T3, which can be affected by nutrition, nonthyroidal hormones, drugs, and illness. 

---