Action of Thyroid Hormones

Primary hyperthyroidism Graves disease Thyroid adenoma/carcinoma Secondary hyperthyroidism 

Hyperthyroidism induced by excessive hypothalamic or pituitary stimulation 

Genetic deficiency of enzymes that synthesize thyroid hormones 

Hypothyroidism induced by peripheral insensitivity to thyroid hormones, inadequate hormone transport, other causes 

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The adult brain is endowed with nuclear as well as cytosolic and membrane T3 receptors that have been visualized by autoradiography and studied biochemically [30-33]. Both neurons and neuropil are labeled by [ 1251]T3, and the labeling is selective across brain regions. Functionally, one of the most prominent features of neural action of thyroid hormone in adulthood is subsensitivity to norepinephrine as a result of a hypothyroid state [27], These changes may be reflections of loss of dendritic spines in at least some neurons of the adultbrain. Clinically, thyroid hormone deficiency increases the probability of depressive illness, whereas thyroid excess increases the probability of mania (Ch. 52) in susceptible individuals [27],... 

Bernal, J. Action of thyroid hormone in brain. /. Endocrin. Investig. 25 268-288,2002. 

Tata JR (2006) Amphibian metamorphosis as a model for the developmental actions of thyroid hormone. Mol Cell Endocrinol 246 10-20... 

Williams GR Neurodevelopment and neurophysiological actions of thyroid hormone. J Neuroendocrinol 2008 20 784. [PMID 18601701]... 

Yen PM, Chin WW. Genomic and nongenomic actions of thyroid hormone. In Braverman LE, Utiger RD, eds. Werner and Ingbar s The Thyroid. 9th ed. New York Lippincott Williams and Wilkins 2005. 

Mousa SA, O Connor LJ, Bergh JJ, et al, The proangiogenic action of thyroid hormone analogue GC-I is initiated at an integrin, J Cardiovasc Pharmacol 2005 46 356-360,... 

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.

Little is known about the mechanism of action of thyroid hormones on microtubule assembly. It might be that, as in other cell types, thyroid hormones regulate the expression of the specific TAU mRNAs and/or their accumulation. One has also to take into account that a large number of other molecules are required to build up neurites during neuronal differentiation (membranes, components of the growth cone, etc.). 

The serum concentration of TSH reflects the integrated action of thyroid hormones at the concentration of one of its target tissues—the pituitary cells that secrete TSH. Thus measurement of TSH is more reliable in the diagnosis of... 

B. Estrogens block the action of thyroid hormones, so the dose of thyroid hormone must be increased. 

The histidine decarboxylase activity of tissues can be raised or lowered by changing the hormonal state of the animal or by subjecting it to certain stressful stimuli. Administration of thyroid hormones to rats produces a marked increase in the specific histidine decarboxylase activity of the glandular mucosa of the stomach " , while the activity of the nonspecific enzyme in the liver is lowered . Studies of the action of thyroid hormones on other pyridoxal-dependent enzymes in rat liver suggest that, in this organ at least, these changes arise from corresponding alterations in both pyridoxal phosphate and apo-enzyme synthesis . 

P.J. Davis, F.B. Davis, Nongenomic actions of thyroid hormone on the heart, Thyroid 2002, 12, 459-466. 

Two distinct genes encode 2 families of TRs, TRa and TRp, and both genes encode multiple receptor isoforms. Certain isoforms (e.g., TR j and TRPj) are found in virtually all thyroid hormone-responsive tissues, whereas TRp is expressed only in the anterior pituitary. Resistance to thyroid hormone has been described in patients with mutations in the TRp gene. In addition to nuclear receptor-mediated actions, there are several well-characterized, nongenomic actions of thyroid hormones, including those occurring at the plasma membrane and on the cellular cytoarchitecture. 

Only those physiologic actions of thyroid hormone that influence fuel metabolism are considered here. It is important to stress the term physiologic, because the effects of supraphysiologic concentrations of thyroid hormone on fuel metabolism may not be simple extensions of their physiologic effects. In general, the following... 

Thyroid hormone binds to receptors in the nucleus that control the expression of genes responsible for many metabolic processes. The T3 receptor exists in two monomeric forms alpha and beta. When activated by T3, the a and (3 monomers combine to form ao, PP, or aP dimers. These Tj-activated dimers bind to DNA response elements and control the synthesis of RNA, which codes for specific proteins that mediate the actions of thyroid hormones. 

Physiological Actions of Thyroid Hormones—Oxygen Consumption and Calorigenesis... 

The two most important actions of thyroid hormone are those related to oxygen consumption and those related to protein synthesis. Most effects of thyroid hormones can be related to the activation of genes following the binding of the hormone to high-affinity receptors of cell nuclei, but direct interactions of thyroid hormones with other cellular receptors cannot be excluded (16). 

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