Endocrinology (from Greek _gr. ἔνδον, "endon", "within"; _gr. κρῑνω, "krīnō", "to separate"; and _gr. -λογία, "-logia") is a branch of medicine dealing with disorder of the
endocrine systemand its specific secretions called hormones.
Function of endocrine organs, hormones and receptors
Hormones are molecules that act as signals from one type of cells to another. Most hormones reach their targets via the blood.
All multicellular organisms need coordinating systems to regulate and integrate the function of cells. Two mechanisms perform this function in higher animals: the nervous system and the endocrine system. The endocrine system acts through the release (generally into the blood) of chemical agents and is vital to the proper development and function of organisms. As Hadley notes, [cite book |author=Hadley, Mac E. |title=Endocrinology |publisher=Prentice Hall |location=Englewood Cliffs, N.J |year=2000 |pages= |edition=5th ed.|isbn=0-13-080356-1 |oclc= |doi= |accessdate=] the integration of developmental events such as proliferation, growth, and differentiation (including
histogenesisand organogenesis) and the coordination of metabolism, respiration, excretion, movement, reproduction, and sensory perception depend on chemical cues, substances synthesised and secreted by specialised cells.
Endocrinology is concerned with the study of the biosynthesis, storage, chemistry, and physiological function of
hormonesand with the cells of the endocrine glands and tissues that secrete them.
The endocrine system consists of several glands, in different parts of the body, that secrete hormones directly into the blood rather ok than into a duct system. 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.
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 also
paracrinemechanisms (chemical communication between cells within a tissue or organ), autocrine signals (a chemical that acts on the same cell), and intracrinesignals (a chemical that acts within the same cell). [cite book |author=Nussey S, Whitehead S |title=Endocrinology: An Integrated Approach |year=2001 |pages= |isbn=1-85996-252-1 |oclc= |doi= |accessdate=] A neuroendocrinesignal is a "classical" hormone that is released into the blood by a neurosecretory neuron (see article on Neuroendocrinology).
Hormones act by binding to specific receptors in the target organ. As Baulieu notes, a receptor has at least two basic constituents:
* a recognition site, to which the hormone binds
* an effector site, which precipitates the modification of cellular function. [cite book |author=Kelly, Paul; Baulieu, Etienne-Emile |title=Hormones: from molecules to disease |publisher=Hermann |location=Paris |year=1990 |pages= |isbn=2-7056-6030-5 |oclc= |doi= |accessdate=]
Between these is a "transduction mechanism" in which hormone binding induces allosteric modification that, in turn, produces the appropriate response.
Chemical classes of hormones
Griffin and Ojeda identify three different classes of hormone based on their chemical composition: [cite book |author=Ojeda, Sergio R.; Griffin, James Bennett |title=Textbook of endocrine physiology |publisher=Oxford University Press |location=Oxford [Oxfordshire] |year=2000 |edition=4th ed. |pages= |isbn=0-19-513541-5 |oclc= |doi= |accessdate=]
Amines, such as
norepinephrine, epinephrine, and dopamine, are derived from single amino acids, in this case tyrosine. Thyroidhormones such as 3,5,3’-triiodothyronine (T3) and 3,5,3’,5’-tetraiodothyronine (thyroxine, T4) make up a subset of this class because they derive from the combination of two iodinated tyrosine amino acid residues.
Peptide and protein
Peptide hormonesand protein hormones consist of three (in the case of thyrotropin-releasing hormone) to more than 200 (in the case of follicle-stimulating hormone) amino acid residues and can have molecular weights as large as 30,000. All hormones secreted by the pituitary gland are peptide hormones, as are leptinfrom adipocytes, ghrelinfrom the stomach, and insulinfrom the pancreas.
Steroid hormonesare converted from their parent compound, cholesterol. Mammalian steroid hormones can be grouped into five groups by the receptors to which they bind: glucocorticoids, mineralocorticoids, androgens, estrogens, and progestagens.
History and key discoveries of endocrinology
The study of endocrinology began in China. The Chinese were isolating sex and pituitary hormones from human urine and using them for medicinal purposes by 200 BC [Temple, Robert. "The Genius of China."pp. 141, 142. ISBN 9781594772177.] . They used many complex methods, such as sublimation. [Temple, Robert. "The Genius of China."p. 142. ISBN 9781594772177] Eventually, when Berthold noted that castrated cockerels did not develop combs and wattles or exhibit overtly male behaviour, European endocrinology began (however, it should be noted that the Chinese anticipated the science by over 1500 years.) [Berthold AA. "Transplantation der Hoden "Arch. Anat. Phsiol. Wiss. Med." 1849;16:42-6.] 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, crystalline
testosteronewas isolated in 1935. [David K, Dingemanse E, Freud J "et al". "Uber krystallinisches mannliches Hormon aus Hoden (Testosteron) wirksamer als aus harn oder aus Cholesterin bereitetes Androsteron. "Hoppe Seylers Z Physiol Chem" 1935;233:281.]
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 classical thinkers such as
Aristotle, Hippocrates, Lucretius, Celsus, and Galen, according to Freeman et al, [cite journal |author=Freeman ER, Bloom DA, McGuire EJ |title=A brief history of testosterone |journal=J. Urol. |volume=165 |issue=2 |pages=371–3 |year=2001 |pmid=11176375 |doi=10.1097/00005392-200102000-00004 |url=] and these theories held sway until the advent of germ theory, physiology, and organ basis of pathology in the 19th century.
In medieval Persia,
Avicenna(980-1037) provided a detailed account on diabetes mellitusin " The Canon of Medicine" (c. 1025), "describing the abnormal appetite and the collapse of sexual functions and he documented the sweet taste of diabetic urine." Like Aretaeus of Cappadociabefore him, Avicenna recognized a primary and secondary diabetes. He also described diabetic gangrene, and treated diabetes using a mixture of lupine, trigonella( fenugreek), and zedoaryseed, which produces a considerable reduction in the excretion of sugar, a treatment which is still prescribed in modern times. Avicenna also "described diabetes insipidus very precisely for the first time", though it was later Johann Peter Frank(1745-1821) who first differentiated between diabetes mellitus and diabetes insipidus.citation|journal=International Journal of Endocrinology and Metabolism|year=2003|volume=1|pages=43–45 [44–5] |title=Clinical Endocrinology in the Islamic Civilization in Iran|last=Nabipour|first=I.]
In the 12th century,
al-Jurjani, another Persian physician, provided the first description of Graves' diseaseafter noting the association of goitreand exophthalmosin his "Thesaurus of the Shah of Khwarazm", the major medical dictionary of its time.WhoNamedIt|synd|1517|Basedow's syndrome or disease - the history and naming of the disease] [citation|journal=Lakartidningen|year=1983|date=August 10, 1983|volume=80|issue=32-33|title=Who was the man behind the syndrome: Ismail al-Jurjani, Testa, Flagani, Parry, Graves or Basedow? Use the term hyperthyreosis instead|last=Ljunggren|first=J. G.|pages=2902 |pmid=6355710 ] Al-Jurjani also established an association between goitre and palpitation. The disease was later named after Irish doctor Robert James Graves, [WhoNamedIt|doctor|695|Robert James Graves] who described a case of goiter with exophthalmos in 1835. The German Karl Adolph von Basedowalso independently reported the same constellation of symptoms in 1840, while earlier reports of the disease were also published by the Italians Giuseppe Flajani and Antonio Giuseppe Testa, in 1802 and 1810 respectively, [WhoNamedIt|doctor|1471|Giuseppe Flajani] and by the English physician Caleb Hillier Parry (a friend of Edward Jenner) in the late 18th century. [cite journal |author=Hull G |title=Caleb Hillier Parry 1755-1822: a notable provincial physician |journal=Journal of the Royal Society of Medicine |volume=91 |issue=6 |pages=335–8 |year=1998 |pmid=9771526 |pmc=1296785 |doi= |url=]
In 1902 Bayliss and Starling performed an experiment in which they observed that acid instilled into the
duodenumcaused the pancreasto begin secretion, even after they had removed all nervous connections between the two. [Bayliss WM, Starling EH. "The mechanism of pancreatic secretion." J Physiol 1902;28:325–352.] The same response could be produced by injecting extract of jejunum mucosainto jugular vein, showing that some factor in the mucosa was responsible. They named this substance " secretin" and coined the term "hormone" for chemicals that act in this way.
Von Mering and Minkowski made the observation in 1889 that removing the
pancreassurgically led to an increase in blood sugar, followed by a coma and eventual death—symptoms of diabetes mellitus. In 1922, Banting and Best realized that homogenizing the pancreas and injecting the derived extract reversed this condition. [cite journal |author=Bliss M |title=J. J. R. Macleod and the discovery of insulin |journal=Q J Exp Physiol |volume=74 |issue=2 |pages=87–96 |year=1989 |pmid=2657840 |doi= |url=] The hormone responsible, insulin, was not discovered until Frederick Sanger sequenced it in 1953. Neurohormones were first identified by Otto Loewiin 1921. [Loewi, O. "Uebertragbarkeit der Herznervenwirkung." Pfluger's Arch. ges Physiol. 1921;189:239-42.] He incubated a frog's heart (innervated with its vagus nerveattached) 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, negative inotropic(beat amplitude) and chronotropic(beat rate) activity were seen in both hearts. This did not occur in either heart if the vagus nerve was stimulated. The vagus nerve was adding something to the saline solution. The effect could be blocked using atropine, a known inhibitor to heart vagal nerve stimulation. Clearly, something was being secreted by the vagus nerve and affecting the heart. The " vagusstuff" (as Loewi called it) causing the myotropiceffects was later identified to be acetylcholineand norepinephrine. Loewi won the Nobel Prize for his discovery.
Recent work in endocrinology focuses on the molecular mechanisms responsible for triggering the effects of hormones. The first example of such work being done was in 1962 by Earl Sutherland. Sutherland investigated whether hormones enter cells to evoke action, or stayed outside of cells. He studied norepinephrine, which acts on the liver to convert
glycogeninto glucosevia the activation of the phosphorylaseenzyme. 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. [cite journal |author=Sutherland EW |title=Studies on the mechanism of hormone action |journal=Science |volume=177 |issue=47 |pages=401–8 |year=1972 |pmid=4339614 |doi= |url=http://www.sciencemag.org/cgi/pmidlookup?view=long&pmid=4339614]
Endocrinology as a profession
Although every organ system secretes and responds to hormones (including the
brain, lungs, heart, intestine, skin, and the kidney), the clinical specialty of endocrinology focuses primarily on the "endocrine organs", meaning the organs whose primary function is hormone secretion. These organs include the pituitary, thyroid, adrenals, ovaries, testes, and pancreas.
An "endocrinologist" is a doctor who specializes in treating disorders of the endocrine system, such as
diabetes, hyperthyroidism, and many others (see list of diseases below).
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.
The diagnosis and treatment of endocrine diseases are guided by
laboratorytests to a greater extent than for most specialties. Many diseases are investigated through "excitation/stimulation" or "inhibition/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 of clinical chemistryand biochemistryto 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 imagingof endocrine organs may reveal incidental findings called incidentalomas, which may or may not represent disease.
Endocrinology involves caring for the person as well as the disease. Most endocrine disorders are
chronic diseases that need life-long care. Some of the most common endocrine diseases include diabetesmellitus, hypothyroidismand the metabolic 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.
Apart from treating patients, many endocrinologists are involved in
clinical scienceand medical research, teaching, and hospital management.
There are roughly 70,000 to 80,000 endocrinologists in the United States. Endocrinologists are specialists of
internal medicineor pediatrics. Reproductive endocrinologists deal primarily with problems of fertilityand menstrual function—often training first in obstetrics. Most qualify as an internist, pediatrician, or gynecologistfor 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, or gynecologyafter 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 3 years of fellowship. Adult endocrinologists are board certified by the American Board of Internal Medicine(ABIM) in Endocrinology, Diabetes and Metabolism.
In North America the principal professional organizations of endocrinologists include The Endocrine Society, [ [http://www.endo-society.org Endo-society.org] ] the American Association of Clinical Endocrinologists, [ [http://www.aace.com AACE.com] ] the American Diabetes Association, [ [http://www.diabetes.org Diabetes.org] ] the Lawson Wilkins Pediatric Endocrine Society, [ [http://www.lwpes.org lwpes.org] ] and the American Thyroid Association. [ [http://www.thyroid.org Thyroid.org] ]
In the United Kingdom, the Society for Endocrinology [ [http://www.endocrinology.org endocrinology.org] ] and the British Society for Paediatric Endocrinology and Diabetes [ [http://www.bsped.org.uk bsped.org.uk] ] are the main professional organisations. The European Society for Paediatric Endocrinology [ [http://www.eurospe.org Eurospe.org] ] is the largest international professional association dedicated solely to paediatric endocrinology. There are numerous similar associations around the world.
Because endocrinology encompasses so many conditions and diseases, there are many organizations that provide education to patients and the public.
The Hormone Foundationis the public education affiliate of The Endocrine Societyand provides information on all endocrine-related conditions. Other educational organizations that focus on one or more endocrine-related conditions include the American Diabetes Association, National Osteoporosis Foundation, Human Growth Foundation, American Menopause Foundation, Inc., and Thyroid Foundation of America.
:See main article at "
Endocrine diseases"A disease due to a disorder of the endocrine system is often called a "hormone imbalance", but is technically known as an "endocrinopathy" or "endocrinosis".
Endocrinologist in Popular Culture
*Dr. Lisa Cuddy , a character on the television show
Elliot Reidbecomes an expert in the field in the Scrubsepisode My Way Home.
* [http://www.ncbi.nlm.nih.gov:80/books/bv.fcgi?call=bv.View..ShowTOC&rid=endocrin.TOC&depth=1 Endocrinology] (British online textbook)
* [http://www.endotext.org Endotext] (American online textbook)
* [http://www.endocrinology.med.ucla.edu/resident.htm Useful Endocrinology Resources for Residents]
* [http://www.intl.elsevierhealth.com/endocrinology/journals.cfm Endocrinology journals from Elsevier]
* [http://www.hormone.org The Hormone Foundation]
ocieties and associations
* [http://www.endo-society.org/ Endocrine Society]
* [http://www.aace.com American Association of Clinical Endocrinologists]
* [http://www.diabetes.org American Diabetes Association]
* [http://www.lwpes.org Lawson Wilkins Pediatric Endocrine Society]
* [http://www.endocrinology.org Society for Endocrinology]
* [http://www.sbn.org Society for Behavioral Neuroendocrinology]
* [http://www.bsped.org.uk British Society for Paediatric Endocrinology & Diabetes]
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