Endocrinology is a branch of medicine dealing with disorders of
the endocrine system and its specific secretions called hormones. An endocrinologist is a doctor who specializes in treating such disorders.
Hormones are molecules that act as signals from one type of cells to another. Those secreted by the endocrine glands travel primarily through 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 on the endocrine organs, i.e. the organs whose primary function is hormone secretion.
Endocrinologists treat disorders of the pituitary, including growth disorders; diseases of the thyroid; diseases of the adrenal glands; diseases of the ovary and testes; and diabetes (although more commonly it is a diabetologist), a disorder of insulin secretion or sensitivity.
As evolution drove the emergence of multicellular organisms it became necessary to develop “coordinating systems to regulate and integrate the function of differentiating cells” (Griffin and Ojeda 2000). Two mechanisms perform this function in higher animals, the nervous system and the endocrine system. Unlike the former, which sends and receives information via electrochemical signals, the endocrine system performs its regulatory function through the detection, production and release (generally into the bloodstream) of chemical agents. The proper function of this system is vital to the proper development of organisms. As Hadley (2000) notes, the integration of developmental events such as proliferation, growth, differentiation (including histogenesis and organogenesis) and the coordination of metabolism, respiration, excretion, movement, reproduction and sensory perception are dependant on “chemical cues, substances synthesised and secreted by the specialised cells within the animal”.
Endocrinology concerns itself with the study of the endocrine system, primarily the biosynthesis, storage, chemistry and physiological function of the ‘chemical cues’ or hormones (a term coined in 1902 – from the Greek όρμάω “I excite” or “I arouse” (Chester-Jones et al 1987)) but also the cells of the endocrine glands and tissues that secrete them. It is also important to consider the mechanism of, and factors controlling, hormone secretion; the mechanisms of hormone action and receptor binding; the pathophysiology of endocrine system dysfunction and the effect of exogenous substances on normal function.
The study of endocrinology effectively began with a study by Berthold (1849), largely ignored at the time, in which he noted that castrated cockerels failed to develop combs and wattles or exhibit overtly male behaviour. It was further noted that the replacement of testes back into the abdominal cavity of either the same bird or another castrated individual resulted in normal behavioural and morphological development. Berthold concluded, rather erroneously, that the testes secreted a substance that conditioned the blood (since they could no longer be nervously enervated) that, in turn, acted upon the body of the cockerel. In fact, one of two other things could have been true based on these data: 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 extensively identified and categorised by early anatomists (and, later, microscopists) a more humoural 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 is comprised of several glands located in diverse areas of the body. Morphologically they are noteworthy because they secrete directly into the bloodstream rather than into a duct system. The hormones produced by these glands can have different functions and modes of actions. One hormone can have multiple effects and target organs and, likewise, one physiological event or target organ can be affected by more than one hormone.
Bayliss and Starling (1902) first conclusively demonstrated the existence of hormones by observing that that acid instilled into the duodenum stimulated secretion by the pancreas, despite the fact that both organs were denervated and, further to this, that the action could be replicated by the intravenous injection of an extract of jejunal mucosa. They termed the substance responsible for this chemical reflex secretin and coined the term hormone for chemicals that act in this manner. As part of this definition they specified that a chemical must be produced by an organ; be released (in small amounts) into the bloodstream; be transported to a distant organ and exert its specific function in order to be classified as a hormone. Although this definition applies to almost all ‘classical’ hormones there also exists paracrine mechanisms (a chemical communication between neighbouring 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 type of ‘classical’ hormone and is released into the bloodstream by a neurosecretory cell upon excited by a nervous signal.
Griffin and Ojeda (2000) identify three different classes of hormone based on their chemical composition:
Amines, such as norepinephrine, epinephrine and dopamine, are derived single amino acids, in this case tyrosine. Thyroid hormones like 3,5,3’-triiodothyronine (T3) and 3,5,3’,5’-tetraiodothyronine (thyroxine, T4) make up a subset of this class as they derive from the combination of two iodinated tyrosine amino acid residues.
Peptide and Protein hormone are comprised of between three (in the case of thryotropin-releasing hormone (glutamic acid–histidine–proline)) and over 200 (in the case of follicle-stimulating hormone) amino acid residues and can have molecular weights as large as 30,000.
Steroid hormones are derivatives of cholesterol and are subdivided into those with an intact steroid nucleus (gonadal and adrenal steroids) and those with a broken steroid nucles (Vitamin D)
Hormones mediate their effects by binding to specific and unique receptors in the target organ or organs, although the response following such recognition varies commensurate with the wide array of differing hormone functions. As Baulieu (1990) notes, though, a receptor consists of at least two basic constituents, to wit, a recognition site to which the hormone binds and an effector (or executive) site, which precipitates the modification of cellular function; between these two entities is a mechanism of transduction in which hormone binding induces allosteric modification that, in turn, produces the appropriate response.
The medical specialty of endocrinology involves the diagnostic evaluation of a wide variety of symptoms and variations, as well as the long-term management of disorders of deficiency or excess of one or more hormones.
The practice of endocrinology is laboratory-oriented. A characteristic of endocrinology is that 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," termed incidentalomas, which do not represent disease.
Endocrinology also requires caring for the person as well as the disease. Most endocrine disorders are chronic diseases warranting life-long medical care. The most common of these is diabetes mellitus. Successful 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 managing patients, many endocrinologists are involved in clinical science and medical research, teaching and hospital management .
Endocrinologists are specialists of internal medicine or pediatrics. Reproductive endocrinologists primarily deal with problems of fertility and menstrual function. Most qualify as an internist, pediatrician, or gynecologist for a few years before specialising, 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 referred to as residency. Further formal training to subspecialize in adult, pediatric, or reproductive endocrinology is referred to as 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.
Among the hundreds of endocrinological diseases are :
Societies and associations
GRIFFIN, J.E. AND OJEDA, S.R., 2000 Textbook of Endocrine Physiology 4th ed. New York: Oxford University Press
HADLEY, M.E., 2000 Endocrinology 5th ed. London: Prentice –Hall International (UK) Ltd.
CHESTER-JONES, I., INGLETON, P.M. AND PHILLIPS, J.G., 1987 Fundamentals of Comparative Vertebrate Endocrinology New York: Plenum Press
BERTHOLD, A.A., 1849 Transplantation der Hoden Arch. Anat. Phsiol. 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 Seylers Z Physiol Chem, 233: 281
FREEMAN, E.R., BLOOM, D.A. AND McGUIRE, J.E., 2001, A Brief History of Testosterone Journal of Urology 165(2):371-373
BAYLISS, W.M. AND STARLING, E.H., 1902 The mechanism of pancreatic secretion. J. Physiol. (London) 28:325–352
NUSSEY, S. AND WHITEHEAD, S., 2001 Endocrinology: An integrated approach Oxford: BIOS Scientific Publishers Ltd.
LAYCOCK, J. AND WISE, P., 1996 Essential Endocrinology Oxford: Oxford University Press
BAULIEU, E-E.,1990 Hormones: From molecules to disease Baulieu, E-E. and Kelly, P.A., (eds) Paris: Hermann