Thyroid Hormones Are Critically Important To Many Bodily Functions
The thyroid gland is a butterfly-shaped organ located at the base of the neck in front of the trachea . Measuring about two inches wide and weighing 20 to 60 grams, the thyroid gland’s function is to make hormones that are vitally important to the bodys metabolism, growth, and development.
In infants, thyroid hormones are crucial to the development of the brain and the skeletal system. A normal functioning thyroid gland is critical to the normal development of children, and to both the long-term and minute-to-minute well-being of adults.
Thyroid Follicles And Thyroid Function
The thyroid is highly vascular, meaning that it has a wealth of blood vessels. It is composed of follicles that absorb iodine, which is needed to produce thyroid hormones. These follicles store iodine and other substances necessary for thyroid hormone production. Surrounding the follicles are folliclar cells. These cells produce and secrete thyroid hormones into circulation via blood vessels. The thyroid also contains cells known as parafollicular cells. These cells are responsible for the production and secretion of the hormone calcitonin.
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.
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Hormone Profiles: Concentrations Over Time
One important consequence of the feedback controls that govern hormone concentrations and the fact that hormones have a limited lifespan or halflife is that most hormones are secreted in “pulses”. The following graph depicts concentrations of luteinizing hormone in the blood of a female dog over a period of 8 hours, with samples collected every 15 minutes:
The pulsatile nature of luteinizing hormone secretion in this animal is evident. Luteinizing hormone is secreted from the anterior pituitary and critically involved in reproductive function the frequency and amplitude of pulses are quite different at different stages of the reproductive cycle.
With reference to clinical endocrinology, examination of the graph should also demonstrate the caution necessary in interpreting endocrine data based on isolated samples.
A pulsatile pattern of secretion is seen for virtually all hormones, with variations in pulse characteristics that reflect specific physiologic states. In addition to the short-term pulses discussed here, longer-term temporal oscillations or endocrine rhythms are also commonly observed and undoubtedly important in both normal and pathologic states.
Functions Of Thyroid Hormones
The thyroid hormones, T3 and T4, are often referred to as metabolic hormones because their levels influence the bodys basal metabolic rate, the amount of energy used by the body at rest. When T3 and T4 bind to intracellular receptors located on the mitochondria, they cause an increase in nutrient breakdown and the use of oxygen to produce ATP. In addition, T3 and T4 initiate the transcription of genes involved in glucose oxidation. These mechanisms prompt cells to produce more ATP which causes an increase in heat production. This so-called calorigenic effect raises body temperature.
Disorders of theEndocrine System: Iodine Deficiency, Hypothyroidism, and Hyperthyroidism
Dietary iodine deficiency can result in the impaired ability to synthesize T3 and T4, leading to a variety of severe disorders. When T3 and T4 cannot be produced, TSH is secreted in increasing amounts. As a result of this hyperstimulation, thyroglobulin accumulates in the thyroid gland follicles, increasing their deposits of colloid. The accumulation of colloid increases the overall size of the thyroid gland, a condition called a goiter . A goiter is only a visible indication of the deficiency. Other symptoms include impaired growth and development, decreased fertility, and prenatal and infant death. Neonatal hypothyroidism is characterized by severe cognitive deficits, short stature, and sometimes deafness and muteness in children and adults born to mothers who were iodine-deficient during pregnancy.
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Iodine For Hormone Production
The thyroid gland produces two primary hormones – thyroxine and tri-iodothyronine . The numbers 3 and 4 refer to the number of atoms of iodine in the hormones. Iodine is essential for the production of thyroid hormones and humans need about 150 mcg each day. Iodine is found in most foods, especially seafood. The soils in Tasmania and along the Great Dividing Range are low in iodine, so the food from these areas can contain insufficient iodine. Iodised salt is the best way to supplement dietary iodine, but taking too much iodine can also be a problem.Of the two hormones produced, T3 is more active than T4, but is produced in much smaller quantities. T4 has a lesser effect, but most is converted to T3 by enzymes that remove one iodine atom. The greater the amount of T3 and T4 circulating in the blood, the faster the metabolism. Lower amounts of T3 and T4 result in a reduced metabolism.
What Do Abnormal T3 Test Results Mean
Because the thyroids functions are complicated, this single test may not give your doctor any definitive answers about what is wrong. However, abnormal results can help point them in the right direction. Your doctor may also choose to perform a T4 or TSH test to gain a clearer picture of your thyroid function.
Abnormally high levels of T3 are common in pregnant women and those with liver disease. If your T3 test also measured the free T3 level, your doctor may be able to rule out these conditions.
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Anatomy Of The Thyroid Gland
The thyroid gland is a butterfly-shaped organ and is composed of two cone-like lobes or wings, lobus dexter and lobus sinister , connected via the isthmus. The organ is situated on the anterior side of the neck, lying against and around the larynx and trachea, reaching posteriorly the oesophagus and carotid sheath.
It starts cranially at the oblique line on the thyroid cartilage , and extends inferiorly to approximately the fifth or sixth tracheal ring. It is difficult to demarcate the glands upper and lower border with vertebral levels because it moves in position in relation to these structures during swallowing.
D Fatty Acid Metabolism
TH stimulates both lipolysis and lipogenesis, although the direct action is lipolysis with lipogenesis thought to be stimulated to restore fat stores . A time course study in rats carefully measured whole body lipid content and thermogenesis after T3 treatment and concluded that the TH-induced lipogenesis is primarily to maintain fat loss that occurs with TH-induced lipolysis . Fatty acids produced from TH-induced lipolysis are the substrate for the increase in thermogenesis . T3 regulation of these divergent metabolic pathways is subject to nuclear receptor crosstalk, ligand-binding, nutritional status, and competition for RXR heterodimers . TH plays a significant role in the conversion of preadipocytes to adipocytes .
Malonyl CoA production in the liver promotes lipogenesis and directly inhibits carnitine palmitoyl transferase -I, which converts long-chain fatty acyl-CoAs to acylcarnitines for translocation from the cytosol into inner mitochondrial matrix where -oxidation occurs . T3 also induces the transcription of acetyl CoA carboxylase -1, which generates malonyl CoA from acetyl CoA. ACC-1 is regulated by TR, LXR, and SREBP-1 . While LXR can directly stimulate ACC-1 , TR and SREBP1 must form a complex that stabilizes SREBP-1 on the binding site . SREBP-1 action is also enhanced by a PPAR agonist, which can potentiate SREBP-1c nuclear activity .
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Effects Of Iodine Deficiency
If there is a deficiency of dietary iodine, the thyroid will not be able to make thyroid hormone. A lack of thyroid hormone will lead to decreased negative feedback on the pituitary, which in turn, will lead to increased production of thyroid-stimulating hormone, which causes the thyroid to enlarge .
This enlarged endemic colloid goiter has the effect of increasing the thyroids ability to trap more iodide, compensating for the iodine deficiency and allowing it to produce adequate amounts of thyroid hormone.
Hormones Produced By The Hypothalamus
The secretion of hormones from the anterior pituitary is controlled by the production of hormones by the hypothalamus. Although there are a number of different hormones they can be split into two main types:
- hormones that tell the pituitary to switch on production of a hormone
- hormones that tell the pituitary to switch off production of a hormone .
The hormones secreted by the posterior pituitary are produced in the hypothalamus and then passed down a tube between the hypothalamus and the pituitary when they are then secreted into the blood.
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Biochemistry Of Thyroid Hormone
Thyroxine and triiodothyronine contain iodine and are formed from thyronines, which are composed of two molecules of the amino acidtyrosine. Thyroxine contains four iodine atoms, and triiodothyronine contains three iodine atoms. Because each molecule of tyrosine binds one or two iodine atoms, two tyrosines are used to synthesize both thyroxine and triiodothyronine. These two hormones are the only biologically active substances that contain iodine, and they cannot be produced in the absence of iodine. The process leading to the eventual synthesis of thyroxine and triiodothyronine begins in the thyroid follicular cells, which concentrate iodine from the serum. The iodine is then oxidized and attached to tyrosine residues within thyroglobulin molecules. The iodinated tyrosine residues are then rearranged to form thyroxine and triiodothyronine. Therefore, thyroglobulin serves not only as the structure within which thyroxine and triiodothyronine are synthesized but also as the storage form of the two hormones.
Why Are Hormones Important
Hormones act as chemical messengers that are released into the blood stream to act on an organ in another part of the body. Although hormones reach all parts of the body, only target cells with compatible receptors are equipped to respond. Over 50 hormones have been identified in humans and other vertebrates.
Hormones control or regulate many biological processes and are often produced in exceptionally low amounts within the body. Examples of such processes include:
- blood sugar control
- differentiation, growth, and function of reproductive organs and estradiol) and
- body growth and energy production .
Much like a lock and key, many hormones act by binding to receptors that are produced within cells. When a hormone binds to a receptor, the receptor carries out the hormone’s instructions, either by altering the cell’s existing proteins or turning on genes that will build a new protein. The hormone-receptor complex switches on or switches off specific biological processes in cells, tissues, and organs.
Some examples of hormones include:
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Can I Live A Normal Life With A Thyroid Disease
A thyroid disease is often a life-long medical condition that you will need to manage constantly. This often involves a daily medication. Your healthcare provider will monitor your treatments and make adjustments over time. However, you can usually live a normal life with a thyroid disease. It may take some time to find the right treatment option for you and control your hormone levels, but then people with these types of conditions can usually live life without many restrictions.
Spotlight On Aging: Thyroid Gland Changes In Older People
Aging itself has only minor effects on the thyroid gland and thyroid hormones. As people get older, the thyroid gland shrinks and shifts lower in the neck. The level of the thyroid hormone triiodothyronine may fall slightly, but the speed of vital functions changes very little. However, thyroid disorders become more common with aging.
Disorders that affect thyroid function, particularly hyperthyroidism and hypothyroidism, can be thought of as great masqueraders in older people. These disorders often cause symptoms that are easily mistaken for symptoms of other conditions or even as signs of getting old.
Increased or decreased thyroid function can dramatically worsen the way an older person feels and can greatly diminish the ability to carry out daily activities. For these reasons, the great masqueraders must be unmasked and recognized for what they are so that they can be effectively treated.
Screening older people for hyperthyroidism and hypothyroidism is helpful. Some experts recommend measuring the level of thyroid-stimulating hormone in the blood in people over 65 every 5 years.
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Treatment For Thyroid Gland Disorders
Problems with thyroid hormone levels can be diagnosed with a simple blood test. The presence of antibodies in the blood will confirm Graves or Hashimotos disease. Underactivity is treated by taking thyroxine tablets – a form of hormone replacement. Overactivity is treated with drugs that slow the activity of the thyroid gland. If these do not work, part or all of the thyroid can be removed surgically, or some or all of the active thyroid cells can be killed with radioactive iodine.Nodules and cancers are diagnosed with a variety of different tests, including ultrasound, special x-rays and fine needle biopsies. Hot nodules will generally be removed surgically or destroyed with radioactive iodine. Cold nodules are frequently left alone and simply kept under observation. Cancer is treated by surgically removing the thyroid gland, followed by treatment with radioactive iodine to destroy any cells which may have spread.Taking iodine supplements can be dangerous for patients with Graves disease or hot nodules.
Thyroxine Iodine And Apoptosis
Thyroxine and iodine stimulate the spectacular apoptosis of the cells of the larval gills, tail and fins in amphibian metamorphosis, and stimulate the evolution of their nervous system transforming the aquatic, vegetarian tadpole into the terrestrial, carnivorous frog. In fact, amphibian frog Xenopus laevis serves as an ideal model system for the study of the mechanisms of apoptosis.
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What Could Go Wrong With My Parathyroid Glands
Sometimes the parathyroid glands make too much parathyroid hormone. In this case, patients may develop a blood level of calcium that is too high , which in turn can make them feel generally unwell however, they may not experience any symptoms. Symptoms may include increased thirst, increased urine production, abdominal pain, constipation, generalised aches and pains, changes in mood. The commonest condition which cause this is called primary hyperparathyroidism. Diagnosis may take some months, as other potential causes of high blood calcium levels need to be excluded. Treatment may include removal of the overactive parathyroid gland or conservative management .See the information sheet on primary hyperparathyroidism for further details.
If the high level of parathyroid hormone remains undetected for a long time, it can cause calcium from the bones to be lost into blood and subsequently the urine. This can eventually cause bones to become thin . Too much calcium in the urine can also cause calcium stones in the kidney.
Occasionally, the parathyroid glands do not produce enough parathyroid hormone leading to low blood calcium levels . This condition is called hypoparathyroidism. This most commonly occurs after neck surgery such as for thyroid disease. Symptoms of low blood calcium include tingling, pins and needle sensations or muscle cramps/spasms. Treatment includes vitamin D or calcium supplementation.
What Is The Function Of The Thyroid Gland
The thyroid gland is part of the endocrine system of the body. Organs of the endocrine system secrete hormones. The primary function of your thyroid gland is to secrete thyroid hormones. The thyroid hormones are involved in regulating many of your bodily functions, such as your breathing, heart rate, temperature, how quickly you burn calories, and digestion, among other functions. Babies and children need adequate amounts of thyroid hormones for brain development and growth. Your thyroid needs iodine in order to manufacture these thyroid hormones. Foods that are naturally rich in iodine include seafood and plants grown in iodine-rich soil. Iodised salt is another good source of dietary iodine.
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Where Are Endocrine Glands Located In The Human Body
Hypothalamus – The hypothalamus links our endocrine and nervous systems together. The hypothalamus drives the endocrine system.
Pituitary gland – The pituitary gland receives signals from the hypothalamus. This gland has two lobes, the posterior and anterior lobes. The posterior lobe secretes hormones that are made by the hypothalamus. The anterior lobe produces its own hormones, several of which act on other endocrine glands.
Thyroid gland – The thyroid gland is critical to the healthy development and maturation of vertebrates and regulates metabolism.
Adrenal glands – The adrenal gland is made up of two glands: the cortex and medulla. These glands produce hormones in response to stress and regulate blood pressure, glucose metabolism, and the body’s salt and water balance.
Pancreas – The pancreas is responsible for producing glucagon and insulin. Both hormones help regulate the concentration of glucose in the blood.
Gonads – The male reproductive gonads, or testes, and female reproductive gonads, or ovaries, produce steroids that affect growth and development and also regulate reproductive cycles and behaviors. The major categories of gonadal steroids are androgens, estrogens, and progestins, all of which are found in both males and females but at different levels.
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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