Endocrinology is a branch of medicine dealing with disorders of the endocrine system and its specific secretions called hormones. Hormones are molecules that act as signals from one type of cells to another. Most hormones reach their targets via the blood. 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 and 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). 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.
All multicellular organisms need â€œCoordinating systems to regulate and integrate the function of differentiating cells.â€ 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 (2000) notes, the integration of developmental events such as proliferation, growth, and differentiation (including histogenesis and organogenesis) and the coordination of metabolism, respiration, excretion, movement, reproduction, and sensory perception depend on â€œchemical cues, substances synthesised and secreted by the specialised cells within the animal.â€
Endocrinology is concerned with the study of the biosynthesis, storage, chemistry, and physiological function of hormones and with the cells of the endocrine glands and tissues that secrete them. The study of endocrinology began when Berthold (1849) noted that castrated cockerels did not develop combs and wattles or exhibit overtly male behaviour. 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 testosterone was isolated in 1935 by David et al.
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 (2001), and these theories held sway until the advent of germ theory, physiology, and organ basis of pathology in the 19th century.
The endocrine system consists of several glands, in different parts of the body that secrete hormones directly into the blood rather 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 1902 Bayliss and Starling performed an experiment in which they observed that acid instilled into the duodenum caused the pancreas to begin secretion, even after they had removed all nervous connections between the two. The same response could be produced by injecting jejunal mucosa, 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. 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 â€˜classicalâ€™ hormones, but there are also paracrine mechanisms (chemical communication between cells within a tissue or organ), autocrine signals (a chemical that acts on the same cell), and intracrine signals (a chemical that acts within the same cell) (Nussey and Whitehead, 2001). A neuroendocrine signal is a â€˜classicalâ€™ 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 (1990) notes, a receptor has at least two basic constituents: a recognition site, to which the hormone binds, and an effector site, which precipitates the modification of cellular function. Between these is a "transduction mechanism" in which hormone binding induces allosteric modification that, in turn, produces the appropriate response.
Griffin and Ojeda (2000) identify three different classes of hormone based on their chemical composition:
Amines, such as nor epinephrine, epinephrine, and dopamine, are derived from single amino acids, in this case tyrosine. Thyroid hormones such as 3,5,3â€™-triiodothyronine (T3) and 3,5,3â€™, 5â€™-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 hormones and 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 leptin from adipocytes, ghrelin from the stomach, and insulin from the pancreas.
Steroid hormones are derived from cholesterol and are subdivided into those with an intact steroid nucleus (gonadal and adrenal steroids) and those with a broken steroid nucleus (vitamin D). Steroid hormones include estrogen and progesterone from the ovary, testosterone from the testes, and cortisol and aldosterone from the adrenal gland.
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 laboratory tests 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 chemistry and biochemistry 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 "spots," called incidentalomas, which do 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. The most common of these is diabetes mellitus. Care of diabetes 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 science and medical research, teaching, and hospital management.
There are roughly 7,000 to 8,000 endocrinologists in the United States. Endocrinologists are specialists of internal medicine or pediatrics. Reproductive endocrinologists deal primarily with problems of fertility and menstrual function - often training first in obstetrics. Most qualify as an internist, pediatrician, or gynecologist 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, or gynecology after medical school is called residency. Further formal training to sub specialize in adult, pediatric endocrinology|, 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.
In North America the principal professional organizations of endocrinologists include The Endocrine Society (www.endo-society.org), the American Association for Clinical Endocrinology (www.aace.com), the American Diabetes Association (www.diabetes.org), the Lawson Wilkins Pediatric Endocrine Society (www.lwpes.org), American Thyroid association ( www.thyroid.org ), and the Academy of Clinical Thyroidologists,(www.thyroidologists.com). In the United Kingdom, the Society for Endocrinology (www.endocrinology.org) and the British Society of Paediatric Endocrinology and Diabetes (www.bsped.org.uk) are the main professional organisations. The European Society of Paediatric Endocrinology  is the largest international professional association dedicated solely to paediatric endocrinology.
There are numerous similar associations around the world.
- Griffin JE, Ojeda SR. Textbook of Endocrine Physiology 4th ed. New York: Oxford University Press, 2000.
- Hadley ME. Endocrinology 5th ed. London: Prentice â€“Hall International (UK) Ltd, 2000.
- Chester-Jones I, Ingleton PM, Phillips JG. Fundamentals of Comparative Vertebrate Endocrinology New York: Plenum Press, 1987.
- Berthold AA. Transplantation der Hoden Arch. Anat. Phsiol. Wiss. Med. 1849;16:42-6.
- 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.
- Freeman ER, Bloom DA, McGuire JE. A Brief History of Testosterone. J Urol 2001;165:371-373.
- Bayliss WM, Starling EH. The mechanism of pancreatic secretion. J Physiol 1902;28:325â€“352.
- Nussey S, Whitehead S. Endocrinology: An integrated approach. Oxford: BIOS Scientific Publishers Ltd., 2001.
- Laylock J, Wise P. Essential Endocrinology Oxford: Oxford University Press, 1996.
- Baulieu EE. Hormones: From molecules to disease Baulieu, E-E. and Kelly, P.A., (eds) Paris: Hermann, 1990.
- Endocrinology (British online textbook)
- Endotext (American online textbook)
- On-Line Endocrinology Journal Club (via JournalReview.org)
- Useful Endocrinology Resources for Residents
Societies and associations
- Endocrine Society
- American Association of Clinical Endocrinologists
- American Diabetes Association
- Lawson Wilkins Pediatric Endocrine Society
- Society for Endocrinology
- Society for Behavioral Neuroendocrinology
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