What Is an Endocrinologist?

An endocrinologist is a medical professional who specializes in diseases and disorders related to hormones, which are chemical messengers that travel throughout the body through the blood stream. Endocrine glands are responsible for the secretion and maintenance of hormones, and they include pituitary, thyroid, and adrenal glands, which all secrete different types of hormones that have differing functions. Hormones control or influence physiological activities such as appetite and metabolism, bone and muscle growth and development, and reproduction.

Endocrinologists are educated and trained to evaluate, diagnose and treat hormone imbalances or dysfunction to restore the equilibrium needed for the body to function properly. Diabetes and thyroid disorders are commonly treated by endocrinologists, and they often work with other specialists to determine the origin of certain disease and disorders. Many diseases and disorders may involve the endocrine system and other systems, such as osteoporosis or infertility.

Endocrinology is a sub-specialty within the specialty field of Internal Medicine. It is a branch of medicine that involves diagnosing and treating diseases and disorders of the endocrine system, including endocrine glands and hormones. Clinical endocrinologists work to identify the actions of certain hormones and the metabolic consequences they have on the body. Physicians in this medical field must be able to classify hormones and their chemical signaling mechanisms, and understand hormone synthesis, secretion and transport, hormone receptors and signal transduction processes, interactions between the endocrine, nervous and immune systems and genetics of endocrinology (Nussey and Whitehead, 2001).

Hormones and the Endocrine System

The endocrine system consists of the following endocrine glands: pineal, hypothalamus, pituitary, thyroid, parathyroid, thymus, adrenals, pancreas, ovaries and testes. Each endocrine gland regulates the secretion of hormones. Hormones communicate and excitement or arousal by their unique chemical structures recognized by specific receptors on their target cells, by their patterns of secretion and their concentrations in the general or localized circulation (Nussey and Whitehead, 2001).

Hormones are categorized into several groups: reproduction and sexual differentiation, development and growth, maintenance or the internal environment, and regulation of metabolism and nutrient supply. Each different function may be affected by one or more hormones, and a single hormone can affect multiple functions.  Hormones act in concert and thus, an abnormality in a controlled variable, such as blood glucose concentration may result from defects in the control of any one of several hormones (Nussey and Whitehead, 2011).  

Clinical endocrinologists typically measure hormone concentrations via urine or saliva testing, although imaging applications such as MR and CT scanning are used as well.

Diseases Treated by an Endocrinologist

Diabetes is one of the most common conditions treated by an endocrinologist. Diabetes is a disease of the pancreas, which regulates and secretes insulin. Inadequate insulin regulation results in high levels of sugar in the blood that can case severe damage to organs, sometimes leading to amputations and even death.

Thyroid disorders may be due to an increased (hyperthyroidism) or decreased production of hormones (hypothyroidism) in the thyroid gland, goiters, or nodules. Hypothalamic disorders, such as abnormal sodium and water balance, and pituitary diseases (tumors, over- or under-production of pituitary hormones) are also treated by an endocrinologist.

Parathyroid abnormalities (hypercalcemia, hypocalcemia), adrenal cortex dysfunction (mineralcorticoid, glucocorticoid, sex hormone abnormalities), gonadal disease (hypogonadism and reproductive disorders), bone metabolism (osteoporosis), lipid metabolism, and iatrogenic effect of glucocorticoids (ACP, 2017).

Other diseases and disorders treated by endocrinologists include (Mahal, 2014):

  • Menopause
  • Infertility
  • Precocious puberty
  • Lack of growth (short stature)
  • Pediatric endocrine diseases
  • Excessive growth or acromegaly/gigantism
  • Cancers of the endocrine glands


The History of Endocrinology


Missed Opportunities

Although he was not aware of the significance of his discovery at the time, Berthold, a German physiologist, was perhaps one of the first to explain endocrinology in 1849. His experimentation on capons (a type of bird) identified that birds with their testes removed were more docile. When he transplanted testes into these birds, they re-developed typically male characteristics. When these birds were autopsied, Berthold noticed the blood supply to the testes was in-tact. Unfortunately, this significant discovery made no impact on advances in science and medicine.

In 1889, Brown-Sequard, age 72, reported to the Academy of Sciences in Paris that he had injected himself with a mixture made up of the blood from testicular veins, semen, and juices of the testicles of dogs or guinea pigs (Wass, 2015). He noted a significant improvement in strength, stamina and the ability to concentrate, although his experiments were not reproduced (Wass, 2015).

Victor Horsley was a famous surgeon who removed the thyroid gland from monkeys and noted the changes replicated those seen in humans with myxoedema. The monkeys without a thyroid experienced hair loss and lethargy. Horsley theorized that sheep thyroids should be transplanted into humans with myxoedema.

The First Thyroid Injection

One of Horsley’s students, George Murray, performed the first to attempt to transplant sheep thyroid tissue to a human. He dissected a sheep thyroid, soaked it in carbolic acid overnight, and filtered it through a handkerchief. This “pink juice” is what he injected into a human female patient. Within 3 months, there was a miraculous improvement in her appearance and her skin was less pale and her energy improved (Wass, 2015). He continued with regular injections with this patient, and she lived nearly 30 years after the first injection.

Identification of Hormones

Up until the late 1800s, the ovary was thought to be part of the nervous system, until Viennese gynecologist Chobrak started removing the ovaries of women in order to treat such conditions as hysteria, anorexia, anxiety and even nymphomania (Wass, 2015). Another physician in Vienna, Joseph Howden, took the removed ovaries and transplanted them to guinea pigs who had their ovaries removed. The transplant reversed the effects the removal of the ovaries had caused. This led to significant discoveries of hormones.

In the early 1900s, Ernest Starling and a colleague were discussing these secretions that could pass through another part of the body and stimulate it directly (Wass, 2015). The Greek word “ormao” was suggested, meaning to excite or arouse, and Starling came up with the word “hormone”.

Many hormones have since been identified, such as the growth hormone in 1932, although the discovery of insulin predated the growth hormone (Wass, 2015). In the late 1800s, it was discovered that removing the pancreas of dogs led to their development of diabetes. The challenge in identifying insulin was that the pancreas secretes exocrine and endocrine compounds, which were difficult to separate at the time.

History of Insulin

The earliest descriptions of diabetes appear in ancient Egyptian texts and ancient Greece as well. Before insulin was used to treat diabetes, fasting and restricted diets were used to treat this disease. The discovery of insulin in the 1920s was one of the most significant medical breakthroughs in the treatment of diabetes. Prior to the discovery of insulin, the life expectancy for children and adults with diabetes was very short.

Frederick Banting was a failed general practitioner in Canada, but he was also the first to extract insulin from a dog pancreas. He tied off the pancreatic duct inside the dog, which led to less exocrine and more endocrine secretion. After 6 weeks, the pancreas was removed, and the insulin was extracted. The first injection of insulin into Leonard Thompson occurred in 1922, when he was 14 and only days away from death (Wass, 2015). He was very thin and smelled of acetone, but the injection of insulin from a dog pancreas saved his life and led to many more lives being saved.

Eli Lilly began to produce insulin using animal pancreases, but they could not standardize the potency or keep up with the high demand. They developed an isoelectric precipitation method that led to more standardization, but didn’t increase the amount of insulin produced. In 1923, a Scandinavian laboratory also began insulin production.

Because the insulin preparation required several injections daily, investigators worked to find ways to prolong its duration of action (Quianzon and Cheikh, 2012). Scientists were able to prolong the action of insulin by adding protamine and zinc, and they were soon incorporated into the animal insulins in production at the time. These insulin doses lasted between 24 and 36 hours, as opposed to the 4-6-hour duration of the additive-free animal insulin. In 1978, DNA technology was incorporated into insulin production technology and became commercialized.

Chronic Complications

As patients with diabetes began to live longer due to the success of insulin treatments, chronic complications of the disease became evident. In 1993, the Diabetes Control and Complications trial demonstrated that the degree of glycemic control was directly related to the severity of complications (Quianzon and Cheikh, 2012). To reduce the incidence of hypoglycemia, which is the major limiting factor for intensive glycemic control, physiologic insulins that mimic the basal and prandial insulin secretion were sought (Quianzon and Cheikh, 2012).

A variety of insulin options are available on the market today, including fast- and short-action and inhalable insulin products. Patients with diabetes do still suffer from complications, but the advances in insulin have led to a significantly increased quality of life for people suffering from this disease.

Other Significant Discoveries

The account of the history of endocrinology would not be complete, however, without mentioning the discovery of leptin and the amazing work of Steve O’Rahilly and Sadaf Farooqi in Cambridge, in examining genetic causes of obesity with such huge success (Wass, 2015).

The Discovery of Leptin

Leptin is a hormone that regulates body weight. In 1950, researchers at The Jackson Laboratory discovered a strain of mice called obese, or ob, with a recessive genetic mutation which caused them to be massively obese: they weighed three times as much as normal mice and have an insatiable appetite (Rockefeller, 2017). A molecular geneticist, Jeffrey Friedman, used positional cloning to identify the specific gene defect that could cause such effects. He was able to successfully identify and clone the ob gene in the mice and in humans. This led to the discovery of the hormone leptin.

Friedman’s discovery led to the identification of a robust physiologic system that regulates food intake and metabolism, that fat is an endocrine organ, and that obesity is a problem of biology (Rockefeller, 2017). Leptin helps the body maintain homeostatic control of its fat mass. It is secreted by fat cells into the bloodstream, until it reaches the brain to regulate appetite and energy release. When the fat mass is reduced, the level of leptin in the blood is reduced, which stimulates appetite and suppressing energy expenditure until the fat mass is restored (Rocekfeller, 2017). Conversely, when the fat mass is increased, levels of leptin increase, and appetite is suppressed until the extra fat mass is lost.

Leptin is used in therapy for patients with obesity. It is also used to treat lipodystrophy, a life-threatening metabolic disorder, as well as other forms of diabetes and hypothalamic ammennorhea (Rockefeller, 2017). Proceeding Freidman’s discovery, tens of thousands of research papers on Leptin have been published.

Leptin, which is made by the body’s fat cells, is the linchpin in a complex endocrine system that not only maintains the body’s weight in a narrow range, but also exerts powerful effects on other aspects of physiology related to an organism’s nutritional state: for example, the entire neuro-endocrine axis including reproduction, glucose metabolism and insulin sensitivity, and immune status (Rockefeller, 2017). In Freidman’s research, the mice lacking leptin had insatiable appetites because their brain perceived that they were starving. When leptin was introduced, the mice ate less and became more active.

Humans with the genetic mutation preventing leptin production may become morbidly obese, although some people with obesity do not have this mutation. Those that do tend to lose weight with leptin therapy. Some people have leptin resistance – their bodies fail to respond to the signals sent by the leptin to control appetite and energy expenditure.

Both laboratory studies and clinical research continues to be done by Freidman and thousands of other scientists and endocrinologists around the world, to determine the mechanisms by which leptin controls feeding behavior and body weight (Rockefeller, 2017). Additional focus on the response of the hypothalamus and neural pathways has suggested that the neural pathways are different in those with obesity and those without.

Genetics of Obesity

Sadaf Farooqi and Stephen O’Rahilly were the first to identify the genetic contribution to obesity. They address both the molecular and clinical aspects of obesity and treating this disease. Genetic syndromes, often involving other disorders such as Down’s syndrome, often involve obesity as a major component of the disease.

How to Become an Endocrinologist

Physicians in this medical field must complete an undergraduate degree, medical school and a residency program.  Most endocrinologists begin their residency in internal medicine and sub-specialize in endocrinology, while others go on to obtain additional training for diabetes or other specialties within the endocrinology medical field through a fellowship program.  

Undergraduate Degree

There is no specific major required for the undergraduate program, although most students major in a pre-medical field or sciences such as chemistry, biology, physics, sociology or humanities, or other sciences. A high GPA and participation in multiple extracurricular activities are typically recommended to prepare for medical school acceptance. Volunteering at a local hospital or medical center should also be considered.

The Medical College Admission Test (MCAT) is taken during the third year of the undergraduate program, in most cases. The MCAT tests students in areas including physical science, critical thinking and problem solving. Because medical school acceptance is extremely competitive, the higher the MCAT score, the better.

Medical School

Future endocrinologists will complete medical school with a Doctorate of Medicine (M.D.) or a Doctor of Osteopathy (D.O.). A D.O. degree typically requires one year of an internship before medical school is completed. The first two years of medical school consists of classroom and laboratory education and training in various subjects including biochemistry, physiology, medical law, pharmacology, and many others.

The final two years typically include rotations in a hospital or clinical setting. Students have the chance to experience a variety of medical specialties including internal medicine, cardiology, pediatrics, and others under the supervision of a licensed physician.

Residency Training

A residency program allows students to receive hands-on experience and direct patient interaction. Most residencies are 3-4 years in length. During a residency program, the physician receives training in internal medicine, pediatrics or obstetrics and gynecology, among many other medical specialties (Mahal, 2014). An endocrinologist must have extensive knowledge of the physiology of the endocrine system, particularly the physiology and biochemistry of hormones, and treatment and management of diseases involving this organ system.

A residency program also consists of seminars, conferences, laboratory research, in addition to patient assessments and evaluations. Some programs require students to have a license before they are accepted, while others do not. Because licensing varies by state, it is important for an endocrinologist to apply for residency programs in the state they wish to practice in.

Internal Medicine Certification

After completion of a residency program, endocrinologists will complete the American Board of Internal Medicine (ABIM). Certification is achieved by meeting the eligibility requirements for accredited medical school and residency programs and passing a written and clinical examination. An ABIM certification is typically required to be accepted into a fellowship program for endocrinology.

Fellowship Program

Most fellowship programs are 2-3 years in duration. The fellowship allows the certified Internalist to focus on and receive specialized training in endocrinology. Students will become familiar with the endocrine system, endocrine glands, and the hormones they secrete. Pediatric endocrinology, diabetes treatment and endocrine pathology are specialties within the endocrinology field that fellows may choose to specialize in.


To legally practice medicine in the United States, all physicians with a M.D. must pass the United States Medical Licensing Exam (USMLE), and those with D.O. status must pass the Comprehensive Osteopathic Medical Licensing Examination (COMLEX). Some states may have additional licensing requirements as well.

Endocrinology Certification

The ABIM offers certification for endocrinologists who have completed a fellowship program. The Endocrinology, Diabetes and Metabolism Exam must be passed to obtain this certification. Endocrinologists must also complete Continuing Medical Education (CME) opportunities to maintain their license and certification.

Specializing in Diabetes

Endocrinologists who wish to specialize in diabetes need additional education and training in the eye, blood vessel, gangrene and amputation of limbs, and kidney diseases associated with diabetes (Mandal, 2014).  Diabetes specialists must also receive training in managing patients of all ages, including pregnant women and those who undergo surgery due to diabetic complications. Endocrinologists who specialize in diabetes must also diagnose and treat obesity, anorexia, lipid disorders, metabolic bone diseases and calcium disorders, and fluid and electrolyte disorders. Laboratory measurement of hormone levels in the blood stream is one of the most important aspects of diagnosing and treating diabetes and other endocrine system diseases and disorders.

Diabetes is one of the most prevalent diseases in the United States, and new advancements in preventing, diagnosing, and treating this disease continue to emerge. The endocrinologist must determine the most efficient and cost-effective means that will provide the highest-quality care to their patients.

Job Prospects

Endocrinologists may practice in a dedicated endocrine practice, as part of a multi-specialty group, or may maintain a split practice seeing both endocrinology and general internal medicine patients. They may also provide consultation for other physicians or hospitals. Academic endocrinologists often participate in basic science or clinical research and teach medical students and resident trainees. ACP, 2017.

The Bureau of Labor Statistics (BLS) projects a growth of 15% by the year 2026 for all physicians and surgeons, including endocrinologists. Because of the prevalence of diabetes and other endocrinology-related diseases, there are a large number of new job opportunities opening up. The median annual salary for an endocrinologist is approximately $208,000, with more than 106,000 new jobs projected. This job growth is much faster than the average for all other occupations.

Due to an aging and growing population, most healthcare careers have a projected increase in employment opportunities through 2026 and beyond.



Mandal, Ananya, MD. What is an Endocrinologist? Medical News Life Sciences, 2014. Retrieved November 15, 2017 from:  https://www.news-medical.net/health/What-is-an-Endocrinologist.aspx

Nussey, S. and Whitehead, S. Endocrinology: An Integrated Approach. BIOS Scientific Publishers, 2001.

Wass, John. The Fantastical World of Hormones. https://www.endocrinology.org/media/1913/the-endocrinologist-115-web-final.pdf

ACP – American College of Physicians. Endocrinology, Diabetes, and Metabolism. Retrieved November 14, 2017 from: https://www.acponline.org/about-acp/about-internal-medicine/subspecialties/endocrinology-diabetes-and-metabolism

Quianzon, C. and Cheikh, I. History of Insulin. J Community Hosp Intern Med Perspect. 2012; 2(2)

Rockefeller University. The Discovery of Leptin, the Hormone that Regulates Body Weight, 2017. Retrieved November 15, 2017 from: http://centennial.rucares.org/index.php?page=Discovery_Leptin

BLS – Bureau of Labor Statistics. Occupational Outlook Handbook, Physicians and Surgeons, 2017. Retrieved November 15, 2017 from: https://www.bls.gov/ooh/healthcare/physicians-and-surgeons.htm

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