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What Is The Target Organ Of Thyroid Hormones Metabolic Effects

The Insulin Receptor Is A Receptor Tyrosine Kinase

Thyroid Physiology: Integrated Effects of Thyroid Hormone

The insulin receptor is found on the surface of all body cells that respond to the hormone. The highest receptor density is found on liver cells in fact, more than half of the insulin that is released by the pancreas is captured by receptors in the liver .

The insulin receptor is located in the cytoplasmic membrane it is a receptor tyrosine kinase. Aside from insulin, human growth hormone and many other growth factors have receptors of this type. Receptor tyrosine kinases are one of the major functional classes of hormone receptors.

A receptor tyrosine kinase has two functional domains. The extracellular domain binds to the hormone. This causes a conformational change to the entire receptor, which activates the intracellular protein tyrosine kinase domain. The activated receptor binds one or several cognate protein substrates, which it then phosphorylates at specific tyrosine residues. The phosphorylated substrates leave the receptor and interact with downstream adapter proteins, which then set off various intracellular signaling cascades.


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D Factors Contributing To Diabetes

TH induces HIF-1 via the PI3K/ERK pathways, as well as by direct induction. The known HIF-1 target genes include the glucose transporter 1 , phosphofructokinase , and monocarboxylate transporter 4 , which regulate cellular glucose metabolism by controlling glucose uptake, glycolysis, and lactate transport, respectively . These genes are induced by physiological doses of T3, and pretreatment with a PI3K inhibitor abolishes this effect . HIF-1 also induces expression of D3 gene leading to reduced T3 and increased rT3 production .

Systemic administration of T1AM rapidly increases endogenous glucose production, glucagon, and corticosterone but does not increase plasma insulin . Central administration of T1AM resulted in a much more profound effect on endogenous glucose production and hyperglucagonemia and reduced plasma insulin . The effects of T1AM on glucose and insulin, like the effects of TH, likely vary with the mode and duration of exposure.

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Vii Conclusion And Future Perspectives

Significant progress has been made in understanding TH targets that mediate metabolic regulation. Several themes have emerged which coordinate these signaling pathways, including nutrient feedback at the cellular and central level, nutrient nuclear receptor crosstalk, local ligand activation, and adrenergic stimulation. This has led to mechanistic insights, especially understanding those factors that modulate multiple TH-regulated pathways. A number of these mechanisms are actively being evaluated as therapeutic targets for metabolic diseases. Although several thyroid hormone analogs have shown significant success in reducing serum LDL cholesterol and producing weight loss, the broad effects of these compounds have limited their clinical application.

B Therapeutic Targets For Metabolic Disorders


An improved understanding of the mechanism underlying the actions of TH on lipid metabolism and thermogenesis has led to several useful compounds targeting TR for treatment of metabolic disorders . The thyroid hormone-related thyronamine signaling is a novel pathway to consider for treatment of obesity and metabolic disturbances . The thyronamines are measurable in normal human sera and tissues . Acute T1AM treatment in animals induces hypothermia and reduces metabolism, similar to torpor in hibernating mammals. Although the factors that regulate endogenous T1AM levels are not known, this is a pathway that could potentially be antagonized to raise metabolic rate. T1AM, however, also has the property of rapidly converting an animal from carbohydrate to exclusive fat metabolism, which persists after acute T1AM stimulation . Selective augmentation of this T1AM action is an attractive target for the treatment of metabolic disorders.

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What Happens If I Have Too Much Thyroxine

The release of too much thyroxine in the bloodstream is known as thyrotoxicosis. This may be caused by overactivity of the thyroid gland , as in Graves’ disease, inflammation of the thyroid or a benign tumour. Thyrotoxicosis can be recognised by a goitre, which is a swelling of the neck due to enlargement of the thyroid gland. Other symptoms of thyrotoxicosis include intolerance to heat, weight loss, increased appetite, increased bowel movements, irregular menstrual cycle, rapid or irregular heartbeat, palpitations, tiredness, irritability, tremor, hair thinning/loss and retraction of the eyelids resulting in a staring appearance.

What Happens If I Have Too Little Thyroid Stimulating Hormone

If a person has too little thyroid stimulating hormone, it is most likely that their thyroid gland is making too much thyroid hormone, that is, they have an overactive thyroid or hyperthyroidism, which is suppressing the thyroid stimulating hormone. People with an overactive thyroid have the opposite symptoms to those with hypothyroidism, i.e. they lose weight , feel too hot and can experience palpitations or anxiety. They may also have a slightly enlarged thyroid gland. Treatment is medication in the form of tablets, which reduce the activity of the thyroid gland and return all thyroid hormone levels to normal. Rarely, problems in the pituitary gland can also result in a low thyroid stimulating hormone, and low free thyroid hormone levels.

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B Role Of Deiodinases

D2 is the primary enzyme responsible for the rapid increases in intracellular T3 in specific tissues as well as the primary producer of serum T3 in humans . The D2 enzyme has a short half-life due to ubiquitination and proteasome degradation . Deubiquitination, which increases D2 activity, is stimulated by adrenergic activation or by low levels of serum T4 . D2 is expressed in key thyroid-responsive tissues, such as brain, skeletal muscle, and brown fat, which preserves T3 in these tissues as serum T4 levels fall. The T3 generated intracellularly by D2 is transferred to the nucleus and then regulates gene transcription. D2 activity is critical for the synergism of TH and signaling in regulating thermogenesis in BAT .

D2 activity has been shown in one study to be stimulated by bile acids, through activation of the G protein-coupled receptor for bile acids receptor, which potentially links TH action with bile acid signaling . Administration of bile acids to mice resulted in increased energy expenditure in BAT, prevented obesity, and improved insulin sensitivity. This action was independent of the FXR but required D2 gene expression. D2 and TGR5 are coexpressed in key metabolic tissues, and this may be relevant for the regulation of energy expenditure. Bile acids may have an action in addition to bile acid homeostasis to function more broadly in metabolism . The TGR5 receptor is expressed in human adipose tissue, and its expression was correlated with basal metabolic rate .

Growth Hormone And Prolactin

Thyroid Hormone Physiology: Thyroid Hormone Metabolism

The functions of growth hormone and prolactin secreted by the pituitary overlap considerably, although prolactin usually regulates water and salt balance, whereas growth hormone primarily influences protein metabolism and hence growth. Prolactin allows migratory fishes such as salmon to adapt from salt water to fresh water. In amphibians, prolactin has been described as a larval growth hormone, and it can also prevent metamorphosis of the larva into the adult. The water-seeking behaviour of adult amphibians often observed prior to breeding in ponds is also controlled by prolactin. The production of a protein-rich secretion by the skin of the discus fish that is used to nourish young offspring is caused by a prolactin-like hormone. Similarly, prolactin stimulates secretions from the crop sac of pigeons , which are fed to newly hatched young. This action is reminiscent of prolactin√Ęs actions on the mammary gland of nursing mammals. Prolactin also appears to be involved in the differentiation and function of many sex accessory structures in nonmammals, and in the stimulation of the mammalian prostate gland. For example, prolactin stimulates cloacal glands responsible for special reproductive secretions. Prolactin also influences external sexual characteristics such as nuptial pads and the height of the tail in male salamanders.

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What Happens If I Have Too Little Thyroxine

Too little production of thyroxine by the thyroid gland is known as hypothyroidism. It may be caused by autoimmune diseases, poor iodine intake or caused by the use of certain drugs. Sometimes, the cause is unknown. Thyroid hormones are essential for physical and mental development so untreated hypothyroidism before birth or during childhood can cause mental impairment and reduced growth.

Hypothyroidism in adults causes reduced metabolism. It can result in symptoms such as fatigue, intolerance of cold temperatures, low heart rate, weight gain, reduced appetite, poor memory, depression, stiffness of the muscles and reduced fertility. See the article on hypothyroidism for more information.

Glucocorticoids And Thyroid Hormones Act On Nuclear Hormone Receptors To Activate Transcription

While they are inactive, nuclear hormone receptors are located in the cytosol. When activated by ligand binding, they translocate to the nucleus and bind to cognate DNA sequences, recruit a number of other regulatory proteins and ultimately induce the transcription of genes in the vicinity of their target DNA sequences.


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Aging Changes In Hormone Production

The endocrine system is made up of organs and tissues that produce hormones. Hormones are natural chemicals produced in one location, released into the bloodstream, then used by other target organs and systems.

Hormones control the target organs. Some organ systems have their own internal control systems along with, or instead of, hormones.

As we age, changes naturally occur in the way body systems are controlled. Some target tissues become less sensitive to their controlling hormone. The amount of hormones produced may also change.

Blood levels of some hormones increase, some decrease, and some are unchanged. Hormones are also broken down more slowly.

Many of the organs that produce hormones are controlled by other hormones. Aging also changes this process. For example, an endocrine tissue may produce less of its hormone than it did at a younger age, or it may produce the same amount at a slower rate.


The hypothalamus is located in the brain. It produces hormones that control the other structures in the endocrine system, including the pituitary gland. The amount of these regulating hormones stays about the same, but the response by the endocrine organs can change as we age.

The pituitary gland is located just below or in the brain. This gland reaches its maximum size in middle age and then gradually becomes smaller. It has two parts:

Insulin is produced by the pancreas. It helps sugar go from the blood to the inside of cells, where it can be used for energy.

Future Directions And Conclusions

summary of thyroid hormone t3 effects on different

Monodeiodination is quantitatively the most important pathway of TH activation. Within peripheral tissue, multiple pathways modulate TH availability. These pathways govern the action and regulation of deiodinase expression, the action of TH transporters, and the expression and crosstalk of TH receptors with multiple partners. This intricate network of TH modifiers increases the sensitivity and the speed of responses to changes induced in the internal and external environment by the thyroid signal. The price to be paid for this is an intricate regulation of each component in time and space. Given the vast spectrum of metabolic body functions regulated by the TH signal, the deiodinases represent a powerful tool with which to modulate cellular metabolism in specific tissues without perturbing systemic levels of THs. Consequently, the development of drugs that target deiodinase action is the next challenge in this field. Extensive work is still required to delineate the kinetics and regulation of the deiodinase enzymes in specific tissues to understand the full spectrum of their biological roles. Thus, pharmacological research is poised to develop deiodinase modulators aimed at driving specific metabolic outcomes. Targeting tissue-specific TH actions may result in novel and safe therapeutic options for metabolic dysfunctions.

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How Is Thyroxine Controlled

The production and release of thyroid hormones, thyroxine and triiodothyronine, is controlled by a feedback loop system that involves the hypothalamus in the brain and the pituitary and thyroid glands. The hypothalamus secretes thyrotropin-releasing hormone which, in turn, stimulates the pituitary gland to produce thyroid stimulating hormone. This hormone stimulates the production of the thyroid hormones, thyroxine and triiodothyronine, by the thyroid gland.

This hormone production system is regulated by a feedback loop so that when the levels of the thyroid hormones increase, they prevent the release of both thyrotropin-releasing hormone and thyroid stimulating hormone. This system allows the body to maintain a constant level of thyroid hormones in the body.

Pituitary: The Master Gland

The pituitary, a pea-sized gland at the base of the brain, produces a number of hormones. Each of these hormones affects a specific part of the body . Because the pituitary controls the function of most other endocrine glands, it is often called the master gland.


* These hormones are produced in the hypothalamus but are stored in and released from the pituitary.

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Effect Of Thyroid Hormones On Body Temperature

Thyroid hormones affect the dilation of blood vessels, which in turn affects the rate at which heat can escape the body. The more dilated blood vessels are, the faster heat can escape.

A person who suffers from hyperthyroidism will experience a fever conversely, a person who suffers from hypothyroidism will experience a decrease in body temperature.

Overview Of Thyroid Hormone Action

Functions of thyroid hormones | Endocrine Physiology mbbs 1st year lectures

Thyroid hormone is produced by the thyroid gland, which consists of follicles in whichthyroid hormone is synthesized through iodination of tyrosine residues in theglycoprotein thyroglobulin . Thyroid stimulating hormone , secreted bythe anterior pituitary in response to feedback from circulating thyroid hormone, actsdirectly on the TSH receptor expressed on the thyroid follicular cellbasolateral membrane . TSH regulates iodideuptake mediated by the sodium/iodide symporter, followed by a series of steps necessaryfor normal thyroid hormone synthesis and secretion . Thyroid hormone is essential for normal development, growth, neuraldifferentiation, and metabolic regulation in mammals and is required for amphibian metamorphosis . These actions are most apparent in conditions of thyroidhormone deficiency during development, such as maternal iodine deficiency or untreatedcongenital hypothyroidism, manifesting as profound neurologic deficits and growthretardation . More subtle and reversibledefects are present when ligand deficiency occurs in the adult .

Nuclear action of thyroid hormone.

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How Hormones Work

Hormones cause changes in target cells by binding to specific cell-surface or intracellular hormone receptors, molecules embedded in the cell membrane or floating in the cytoplasm with a binding site that matches a binding site on the hormone molecule. In this way, even though hormones circulate throughout the body and come into contact with many different cell types, they only affect cells that possess the necessary receptors. Receptors for a specific hormone may be found on or in many different cells or may be limited to a small number of specialized cells. For example, thyroid hormones act on many different tissue types, stimulating metabolic activity throughout the body. Cells can have many receptors for the same hormone but often also possess receptors for different types of hormones. The number of receptors that respond to a hormone determines the cells sensitivity to that hormone, and the resulting cellular response. Additionally, the number of receptors available to respond to a hormone can change over time, resulting in increased or decreased cell sensitivity. In up-regulation, the number of receptors increases in response to rising hormone levels, making the cell more sensitive to the hormone and allowing for more cellular activity. When the number of receptors decreases in response to rising hormone levels, called down-regulation, cellular activity is reduced.

What Is A Hormone

A hormone is a regulatory biochemical produced by the glands and taken via the blood to different organs to regulate its physiology and behaviour. In that regard then you can think of it as an instruction that tells the organ to behave in a particular way and thus regulate a number of important biological functions.

These biological functions include:

  • Metabolism
  • And more

While the hormones are created in the glands, many of them are regulated by the brain. Hormones can also act as neurotransmitters for instance melatonin is both a hormone and a neurotransmitter.

Different hormones are made in different ways. Protein hormones and catecholamines are types of hormones that are water soluble and thus can be easily carried around the circulatory system. Other hormones though such as steroid and thyroid hormones are lipid soluble . Steroid and thyroid hormones must bond to plasma glycoproteins which transport them.

In order to affect the target organs, hormones must bond to the target tissue via receptor proteins. These are proteins that are designed to attract and capture those specific hormones and will only respond to the correct hormones. This is similar to the way that neurotransmitters must be received by specific receptors in order to be effective.

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