The endocrine system consists of severalglands, all in different parts of the body, that secrete hormones directly into the blood rather than into aduct system. Therefore,endocrine glands are regarded as ductless glands. Hormones have many different functions and modes of action; one hormone may have several effects on different target organs, and, conversely, one target organ may be affected by more than one hormone.
Endocrinology is the study of the endocrine system in thehuman body.[2] This is a system ofglands which secrete hormones. Hormones are chemicals that affect the actions of different organ systems in the body. Examples includethyroid hormone,growth hormone, andinsulin. The endocrine system involves a number of feedback mechanisms, so that often one hormone (such asthyroid stimulating hormone) will control the action or release of another secondary hormone (such asthyroid hormone). If there is too much of the secondary hormone, it may provide negative feedback to the primary hormone, maintaininghomeostasis.[3][4][5]
In the original 1902 definition by Bayliss and Starling (see below), they specified that, to be classified as a hormone, a chemical must be produced by an organ, be released (in small amounts) into the blood, and be transported by the blood to a distant organ to exert its specific function. This definition holds for most "classical" hormones, but there are alsoparacrine mechanisms (chemical communication between cells within a tissue or organ),autocrine signals (a chemical that acts on the same cell), andintracrine signals (a chemical that acts within the same cell).[6] Aneuroendocrine signal is a "classical" hormone that is released into the blood by a neurosecretory neuron (see article onneuroendocrinology).[citation needed]
Although every organ system secretes and responds to hormones (including thebrain,lungs,heart,intestine,skin, and thekidneys), the clinical specialty of endocrinology focuses primarily on theendocrine organs, meaning the organs whose primary function is hormone secretion. These organs include thepituitary,thyroid,adrenals,ovaries,testes, andpancreas.
The medical specialty of endocrinology involves the diagnostic evaluation of a wide variety of symptoms and variations and the long-term management of disorders of deficiency or excess of one or more hormones.[9]
The diagnosis and treatment of endocrine diseases are guided bylaboratory tests to a greater extent than for most specialties. Many diseases are investigated throughexcitation/stimulation orinhibition/suppression testing. This might involve injection with a stimulating agent to test the function of an endocrine organ. Blood is then sampled to assess the changes of the relevant hormones or metabolites. An endocrinologist needs extensive knowledge ofclinical chemistry andbiochemistry to understand the uses and limitations of the investigations.
A second important aspect of the practice of endocrinology is distinguishing human variation from disease. Atypical patterns of physical development and abnormal test results must be assessed as indicative of disease or not.Diagnostic imaging of endocrine organs may reveal incidental findings calledincidentalomas, which may or may not represent disease.[10]
Endocrinology involves caring for the person as well as the disease. Most endocrine disorders arechronic diseases that need lifelong care. Some of the most common endocrine diseases includediabetes mellitus,hypothyroidism and themetabolic syndrome. Care of diabetes, obesity and other chronic diseases necessitates understanding the patient at the personal and social level as well as the molecular, and the physician–patient relationship can be an important therapeutic process.
Endocrinologists are specialists ofinternal medicine orpediatrics. Reproductive endocrinologists deal primarily with problems offertility and menstrual function—often training first in obstetrics. Most qualify as aninternist,pediatrician, orgynecologist for a few years before specializing, depending on the local training system. In the U.S. and Canada, training for board certification in internal medicine,pediatrics, orgynecology after medical school is called residency. Further formal training to subspecialize in adult,pediatric, or reproductive endocrinology is called a fellowship. Typical training for a North American endocrinologist involves 4 years of college, 4 years of medical school, 3 years of residency, and 2 years of fellowship. In the US, adult endocrinologists are board certified by theAmerican Board of Internal Medicine (ABIM) or theAmerican Osteopathic Board of Internal Medicine (AOBIM) in Endocrinology, Diabetes and Metabolism.[citation needed]
Diabetes mellitus: This is a chronic condition that affects how your body regulatesblood sugar. There are two main types:type 1 diabetes, which is an autoimmune disease that occurs when the body attacks the cells that produce insulin, andtype 2 diabetes, which is a condition in which the body either doesn't produce enough insulin or doesn't use it effectively.
Thyroid disorders: These are conditions that affect the thyroid gland, a butterfly-shaped gland located in the front of your neck. The thyroid gland produceshormones that regulate your metabolism, heart rate, andbody temperature. Common thyroid disorders include hyperthyroidism (overactive thyroid) and hypothyroidism (underactive thyroid).
Adrenal disorders: The adrenal glands are located on top of yourkidneys. They produce hormones that help regulate blood pressure, blood sugar, and the body's response to stress. Commonadrenal disorders include Cushing syndrome (excess cortisol production) and Addison's disease (adrenal insufficiency).
Pituitary disorders: The pituitary gland is a pea-sized gland located at the base of the brain. It produces hormones that control many other hormone-producing glands in the body. Commonpituitary disorders include acromegaly (excess growth hormone production) and Cushing's disease (excess ACTH production).
Metabolic disorders: These are conditions that affect how your body processes food into energy. Commonmetabolic disorders include obesity, highcholesterol, and gout.
Calcium and bone disorders:Endocrinologists also treat conditions that affect calcium levels in the blood, such as hyperparathyroidism (too much parathyroid hormone) and osteoporosis (weak bones).
Sexual andreproductive disorders: Endocrinologists can also help diagnose and treat hormonal problems that affectsexual development and function, such as polycystic ovary syndrome (PCOS) and erectile dysfunction.
Endocrine cancers: These are cancers that develop in the endocrine glands. Endocrinologists can help diagnose and treat thesecancers.
Endocrinology also involves the study of the diseases of the endocrine system. These diseases may relate to too little or too much secretion of a hormone, too little or too much action of a hormone, or problems with receiving the hormone.
Because endocrinology encompasses so many conditions and diseases, there are many organizations that provide education to patients and the public.The Hormone Foundation is the public education affiliate ofThe Endocrine Society and provides information on all endocrine-related conditions. Other educational organizations that focus on one or more endocrine-related conditions include theAmerican Diabetes Association,Human Growth Foundation, American Menopause Foundation, Inc., and American Thyroid Association.[11][12]
In North America the principal professional organizations of endocrinologists include The Endocrine Society,[13] theAmerican Association of Clinical Endocrinologists,[14] the American Diabetes Association,[15] the Lawson Wilkins Pediatric Endocrine Society,[16] and the American Thyroid Association.[17]
In Europe, the European Society of Endocrinology (ESE) and the European Society for Paediatric Endocrinology (ESPE) are the main organisations representing professionals in the fields of adult and paediatric endocrinology, respectively.
In the United Kingdom, theSociety for Endocrinology[18] and the British Society for Paediatric Endocrinology and Diabetes[19] are the main professional organisations.
The European Society for Paediatric Endocrinology[20] is the largest international professional association dedicated solely to paediatric endocrinology. There are numerous similar associations around the world.
The earliest study of endocrinology began in China.[21] The Chinese were isolating sex and pituitary hormones from humanurine and using them for medicinal purposes by 200 BC.[21] They used many complex methods, such as sublimation of steroid hormones.[21] Another method specified by Chinese texts—the earliest dating to 1110—specified the use ofsaponin (from the beans ofGleditsia sinensis) to extract hormones, butgypsum (containingcalcium sulfate) was also known to have been used.[21]
Although most of the relevant tissues and endocrine glands had been identified by early anatomists, a more humoral approach to understanding biological function and disease was favoured by theancient Greek andRoman thinkers such asAristotle,Hippocrates,Lucretius,Celsus, andGalen, according to Freeman et al.,[22] and these theories held sway until the advent ofgerm theory, physiology, and organ basis of pathology in the 19th century.
In 1849,Arnold Berthold noted that castrated cockerels did not develop combs and wattles or exhibit overtly male behaviour.[23] He found that replacement of testes back into the abdominal cavity of the same bird or another castrated bird resulted in normal behavioural and morphological development, and he concluded (erroneously) that the testes secreted a substance that "conditioned" the blood that, in turn, acted on the body of the cockerel. In fact, one of two other things could have been true: that the testes modified or activated a constituent of the blood or that the testes removed an inhibitory factor from the blood. It was not proven that the testes released a substance that engenders male characteristics until it was shown that the extract of testes could replace their function in castrated animals. Pure, crystallinetestosterone was isolated in 1935.[24]
In 1902William Bayliss andErnest Starling performed an experiment in which they observed that acid instilled into theduodenum caused thepancreas to begin secretion, even after they had removed all nervous connections between the two.[29] The same response could be produced by injecting extract of jejunum mucosa into the jugular vein, showing that some factor in the mucosa was responsible. They named this substance "secretin" and coined the termhormone for chemicals that act in this way.
Joseph von Mering andOskar Minkowski made the observation in 1889 that removing thepancreas surgically led to an increase inblood sugar, followed by a coma and eventual death—symptoms ofdiabetes mellitus. In 1922, Banting and Best realized that homogenizing the pancreas and injecting the derived extract reversed this condition.[30]
Neurohormones were first identified byOtto Loewi in 1921.[31] He incubated a frog's heart (innervated with itsvagus nerve attached) in a saline bath, and left in the solution for some time. The solution was then used to bathe a non-innervated second heart. If the vagus nerve on the first heart was stimulated, negativeinotropic (beat amplitude) andchronotropic (beat rate) activity were seen in both hearts. This did not occur in either heart if the vagus nerve was not stimulated. The vagus nerve was adding something to the saline solution. The effect could be blocked usingatropine, a knowninhibitor toheart vagal nerve stimulation. Clearly, something was being secreted by thevagus nerve and affecting the heart. The "vagusstuff" (as Loewi called it) causing the myotropic (muscle enhancing) effects was later identified to beacetylcholine andnorepinephrine. Loewi won the Nobel Prize for his discovery.
Recent work in endocrinology focuses on the molecular mechanisms responsible for triggering the effects ofhormones. The first example of such work being done was in 1962 byEarl Sutherland. Sutherland investigated whether hormones enter cells to evoke action, or stayed outside of cells. He studiednorepinephrine, which acts on the liver to convertglycogen intoglucose via the activation of thephosphorylase enzyme. He homogenized the liver into a membrane fraction and soluble fraction (phosphorylase is soluble), added norepinephrine to the membrane fraction, extracted its soluble products, and added them to the first soluble fraction. Phosphorylase activated, indicating that norepinephrine's target receptor was on the cell membrane, not located intracellularly. He later identified the compound as cyclic AMP (cAMP) and with his discovery created the concept of second-messenger-mediated pathways. He, like Loewi, won the Nobel Prize for his groundbreaking work in endocrinology.[32]
^Carroll, Robert G. (2007-01-01), Carroll, Robert G. (ed.),"13 - Endocrine System",Elsevier's Integrated Physiology, Philadelphia: Mosby, pp. 157–176,ISBN978-0-323-04318-2, retrieved2023-11-15
^abcdTemple, Robert (2007) [1986].The genius of China: 3,000 years of science, discovery & invention (3rd ed.). London: Andre Deutsch. pp. 141–145.ISBN978-0-233-00202-6.
^Berthold AA (1849). "Transplantation der Hoden".Arch. Anat. Physiol. Wiss. Med.16:42–6.
^David K; Dingemanse E; Freud J; et al. (1935). "Uber krystallinisches mannliches Hormon aus Hoden (Testosteron) wirksamer als aus harn oder aus Cholesterin bereitetes Androsteron".Hoppe-Seyler's Z Physiol Chem.233 (5–6):281–283.doi:10.1515/bchm2.1935.233.5-6.281.
