18.8: Introduction to Endocrine Glands - Biology

Describe the role of different glands in the endocrine system

Both the endocrine and nervous systems use chemical signals to communicate and regulate the body’s physiology. Because the neurons can regulate the release of hormones, the nervous and endocrine systems work in a coordinated manner to regulate the body’s physiology.

What You’ll Learn to Do

  • Describe the role of the hypothalamic-pituitary axis in the endocrine system
  • Describe the role of the thyroid and parathyroid glands in the endocrine system
  • Describe the role of the adrenal gland in the endocrine system
  • Describe the role of the pancreas, pineal gland and gonads in the endocrine system
  • Identify different organs that have secondary endocrine functions

Learning Activities

The learning activities for this section include the following:

  • Hypothalamic-Pituitary Axis
  • Thyroids
  • Adrenal Glands
  • The Pancreas, Pineal Gland, and Gonads
  • Organs with Secondary Endocrine Functions
  • Self Check: Endocrine Glands

18.8: Introduction to Endocrine Glands - Biology

What is the System?

  1. Made up of glands that produce and secrete hormones, _________________________
  2. Regulation of growth, metabolism, and ______________________________
  3. Responses to ________________________________
  4. Maintains _____________________________________

Major Structures & Location

Hypothalamus (part of the brain)

1. Pineal
2. Pituitary
3. Thyroid & Parathyroid

4. Thymus
5. Adrenals
6. Pancreas
7. Ovary
8. Testes

Types of Glands

Control of Hormonal Secretions - Negative versus Positive Feedback

Negative Feedback

When the levels go above or below a _______________________, the endocrine system secretes hormones to lower or raise the level.

Positive Feedback

Pituitary Gland

Why is it called the master gland?

What part of the brain controls it?

Anterior Pituitary Hormones

Prolactin or PRL –
Growth hormone or GH
Adrenocorticotropin or ACTH –
Thyroid-stimulating hormone or TSH -.
Luteinizing hormone or LH –

Follicle-stimulating hormone or FSH

Posterior Pituitary Hormones

Antidiuretic hormone or ADH

Thyroid Gland

The thyroid hormones control your _________________________, which is the body's ability to break down food and store it as energy

Thyroxin (T4) & Tri-iodothyronine (T3) - increase the rate at which cells release energy from carbohydrates
Calcitonin – regulates the blood concentration of calcium

Hypothyroidism (cretinism)
Hyperthyroidism (Grave&rsquos disease)

Parathyroid Gland

Located behind the thyroid, four tiny glands that help maintain calcium and phosphorous levels

Parathyroid Hormone (PTH) - takes calcium from the bones to make it available in the blood

Adrenal Glands Located above each kidney.

Adrenal Cortex = ______________ area Medulla = ______________

Adrenal glands produce _______________________________

Epinephrine & Norepinephrine – increased heart rate, breathing rate, elevated blood pressure (fight or flight, response to stress)
Aldosterone –helps kidneys conserve sodium and excrete potassium, maintaining ___________________
Cortisol – glucocortoid, keeps blood glucose levels stable response to ___________
Adrenal Sex Hormones - androgens (male) and estrogens (female)

Adrenal Gland Disorders

Large gland behind stomach, maintains healthy blood sugar (glucose) levels.
Contains islands of cells called the Islets of Langerhans which secrete glucagon and insulin

Glucagon – stimulates the liver to break down glycogen, Raises ______________________________________

Insulin – decreases blood sugar concentrations, affects the ____________________ of glucose by cells
Diabetes Mellitus –insulin deficiency, blood sugar rises (hyperglycemia) and excess is excreted in the urine

What is a diabetic neuropathy?

Other Endocrine Glands

Pineal Gland – secretes melatonin which maintains _____________________________

Thymus Gland – large in young children, gradually shrinks with age, secretes thymosins, important to ______________________

Reproductive Glands – testes and ovaries – testosterone, progesterone, estrogen
What is gonadotropin?

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Thyroid Gland Function

The thyroid gland has the main role in controlling our body’s metabolism. The simplest way to define metabolism is our body’s ability to convert food into energy. This “fuel” is burned at different rates depending on the person, which is why people are said to have a “fast” or “slow” metabolism. Furthermore, the thyroid will secrete the hormones that will regulate our vitals and maintain our internal homeostasis. Among the most common, primitive functions of the body it controls are our breathing and our heart rate.

Our weight is likewise monitored by the thyroid gland, which explains why patients with a compromised thyroid gland will have a weight that fluctuates drastically, as we will discuss in more detail later. Even our internal body temperature and our cholesterol levels will be finely tuned with the help of thyroid hormone release.

What is an endocrinologist?

After completing four years of medical school, people who want to be endocrinologists then spend three or four years in an internship and residency program. These specialty programs cover internal medicine, pediatrics, or obstetrics and gynecology, according to the American Board of Internal Medicine.

Endocrinologists-in-training then spend two or three more years learning how to diagnose and treat hormone conditions. Overall, an endocrinologist's training will take more than 10 years after the undergraduate degree. They are certified by the American Board of Internal Medicine. Endocrinologists typically specialize in one or two areas of endocrinology, such as diabetes or infertility. These specialists treat patients with fertility issues and also assess and treat patients with health concerns surrounding menstruation and menopause, Loh noted.

Endocrine Controls

To control endocrine functions, the secretion of each hormone must be regulated within precise limits. The body is normally able to sense whether more or less of a given hormone is needed.

Many endocrine glands are controlled by the interplay of hormonal signals between the hypothalamus, located in the brain, and the pituitary gland, which sits at the base of the brain. This interplay is referred to as the hypothalamic-pituitary axis. The hypothalamus secretes several hormones that control the pituitary gland.

The pituitary gland, sometimes called the master gland, in turn controls the functions of many other endocrine glands. The pituitary controls the rate at which it secretes hormones through a feedback loop in which the blood levels of other endocrine hormones signal the pituitary to slow down or speed up. So, for example, the pituitary gland senses when blood levels of thyroid hormone are low and releases thyroid stimulating hormone, which tells the thyroid gland to make more hormones. If the level gets too high, the pituitary senses that and decreases the amount of thyroid stimulating hormone, which then decreases the amount of thyroid hormone produced. This back-and-forth adjustment (feedback) keeps hormone levels in proper balance.

Many other factors can control endocrine function. For example, a baby sucking on its mother's nipple stimulates her pituitary gland to secrete prolactin and oxytocin , hormones that stimulate breast milk production and flow. Rising blood sugar levels stimulate the islet cells of the pancreas to produce insulin . Part of the nervous system stimulates the adrenal gland to produce epinephrine .

Lesson Unlocking the Endocrine System

Units serve as guides to a particular content or subject area. Nested under units are lessons (in purple) and hands-on activities (in blue).

Note that not all lessons and activities will exist under a unit, and instead may exist as "standalone" curriculum.

  • Engineering and the Human Body
    • Spaced Out
    • Move Your Muscles!
      • Walk, Run, Jump!
      • Muscles, Muscles Everywhere
      • Our Amazing Skeleton
        • Fascinating Friction!
        • Digestive System
          • Design Devices to Help Astronauts Eat: Lunch in Outer Space!
          • The Heart of the Matter
            • Blood Cell Basics
            • The Beat Goes On
            • Do You Have the Strength?
            • Nerve Racking
              • 20/20 Vision
              • Sound Line
              • Engineering a Mountain Rescue Litter
              • Unlocking the Endocrine System
                • Endocrine Excitement!
                • Just Passing Through
                  • Kidney Filtering
                  • Out of Breath
                    • Creating Model Working Lungs: Just Breathe
                    • Fighting Back!
                      • Hot or Not

                      TE Newsletter

                      The Endocrine System


                      Engineering Connection

                      The endocrine system helps us learn the importance of communication in the body. Good communication skills are also an important part of engineering. Astronauts have to communicate well with each other both on Earth and in outer space. Engineers also design the technologies that make communication in space and on Earth possible, including cell phones, digital video equipment and satellites.

                      Learning Objectives

                      After this lesson, students should be able to:

                      • List several parts of the endocrine system.
                      • Compare the endocrine system to a mail delivery system.
                      • Explain why communication is important for engineers and astronauts.

                      Educational Standards

                      Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

                      All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (

                      In the ASN, standards are hierarchically structured: first by source e.g., by state within source by type e.g., science or mathematics within type by subtype, then by grade, etc.

                      NGSS: Next Generation Science Standards - Science

                      MS-LS1-3. Use argument supported by evidence for how the body is a system of interacting subsystems composed of groups of cells. (Grades 6 - 8)

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                      Scientists and engineers are guided by habits of mind such as intellectual honesty, tolerance of ambiguity, skepticism, and openness to new ideas.

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                      International Technology and Engineering Educators Association - Technology

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                      State Standards
                      Colorado - Science
                      • Analyze and interpret data to generate evidence that human systems are interdependent (Grade 5) More Details

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                      More Curriculum Like This

                      In this activity, students are divided into a group of hormones and a group of receptors. The hormones have to find their matching receptors, and the pair, once matched, perform a given action. This activity helps students learn about the specificity of hormone-receptor interactions within the endoc.


                      Today we are going to talk about communication. Who can give me a definition of what communication is? (Possible answers: Communication involves talking to other people, conveying information between people, etc.). Great job! Thank you for thinking hard about that. Now, we have been talking a lot about astronauts and outer space, so let's think together about why communication would be important for astronauts. Does anyone have any ideas? (Possible answers: Astronauts need to be able to talk with each other, even when they are in their space suits, and the astronauts in the space shuttle need to be able to talk back and forth with mission control on Earth). Great answers! Now let's talk about one more group of people that need to be really good at communicating: engineers! Why do you think it is so important for engineers to be good communicators? Engineers must be able to explain their ideas so that other people can understand them.

                      How does this relate to the human body? Well, today we are going to learn about a body system that is all about communication! This system is called the endocrine system (write the word endocrine on the board). The endocrine system helps carry messages throughout your body, to tell your body what to do. You can think of it as a giant mail system.

                      Here is how it works: your body has many endocrine glands, which secrete hormones into your blood. The bloodstream carries the hormones to a specific place (an organ or a receptor) that is designed to receive them. Once the hormone gets to that specific place, it gives your body some special instructions. Some of these instructions tell your body to make more red blood cells, to make more white blood cells, to secrete acid to digest food, to absorb calcium, or even to make you not feel hungry any more. Hormones can also tell the cells in your body when to divide and grow.

                      So, if we compare this whole endocrine system to how mail gets delivered, the endocrine gland would be like someone who puts a letter in the mailbox, then the bloodstream (which would be like the mail carrier) carries the letter to exactly where it is supposed to go (to just the right new mailbox, which would be like an organ, or receptor). Then, when the person who receives the mail reads their letter, it is similar to your body receiving the hormone (at the organ or receptor) and then doing what the hormone (letter) suggests to do. Pretty neat, isn't it!

                      In a microgravity environment such as space, astronauts cannot easily send letters back to Earth to see how everything is going. However, astronauts need to be able to communicate with ground control on Earth to see if their body systems are being monitored correctly and even if the timing is right for their return to Earth. Engineers need to understand how to best communicate in return with the astronauts as well, and they work to design the technologies, including cameras, video equipment, satellite phones and monitoring equipment, to be able to communicate with the astronauts while they are so far from home.

                      Lesson Background and Concepts for Teachers

                      The endocrine system is all about communication. There are two main communication pathways in your body: the nervous system and the endocrine system. In the nervous system, signals travel very fast, and lead to almost instantaneous responses. In the endocrine system, chemicals travel through your body more slowly, and the response to these chemicals can be slow and/or long lasting.

                      What is a hormone? It is a chemical that has a high level of specificity, which means that it will only react with a specific receptor site in your body. The lock and key analogy is often used to explain this specificity, and it is a great way to think about how hormones work. Hormones convey important information to the body, including such instructions as cell division and growth, appetite suppression, acid secretion, calcium absorption, and red and white blood cell production. Students can learn more about hormone interactions with the associated activity Endocrine Excitement!

                      Hormones are secreted by endocrine glands. There are eight major endocrine glands. Those glands, along with their main functions, are listed below:

                      Pituitary gland – regulates other endocrine glands secretes growth hormone.

                      Thyroid – regulates metabolic rate.

                      Thymus – assists in development of immune system.

                      Adrenal gland – regulates fluid and sodium balance emergency warning system under stress.

                      Ovary – controls development of secondary sex characteristics and functioning of sex organs.

                      Testis - controls development of secondary sex characteristics and functioning of sex organs.

                      Pancreatic islets – helps regulate blood sugar.

                      Pineal gland - believed to regulate biorhythms and moods and stimulate the onset of puberty.

                      Figure 1. Major endocrine glands.

                      Two hormones that engineers are involved in producing are growth hormone and insulin. Growth hormone can be used for children (or some adults) whose bodies do not produce enough on their own, and insulin is needed for people who have diabetes.

                      Associated Activities

                      • Endocrine Excitement! - Students create hormone-receptor pairs by matching puzzle pieces and then follow the instructions written on the pieces.

                      Lesson Closure

                      Today we learned about the endocrine system and how it helps the body communicate signals like when to grow or digest food. Who can tell me the four main parts of the endocrine system? (Answer: endocrine glands, hormones, receptor sites, bloodstream) How is the endocrine system like the mail system? Well, the endocrine gland sends a hormone message, like a letter, and the bloodstream mail carrier carries it to the receptor site, like a new mailbox. Lastly, the body reacts to the hormone message, as somebody might if they read the letter. It's all about communication!

                      Who remembers why communication is important to astronauts and engineers? That's right, astronauts and engineers have to communicate well with each other both on Earth and in outer space. Engineers also design the technologies that make communication in space and on Earth possible, including cell phones, digital video equipment and satellites.


                      Adrenal Gland: Regulates fluid and sodium balance emergency warning system under stress.

                      Endocrine Gland: A gland in the body which secretes hormones into the bloodstream.

                      Hormone: A chemical secreted by endocrine glands which carries instructions to the body.

                      Ovary: Controls development of secondary sex characteristics and functioning of sex organs.

                      Pancreatic islets: Helps regulate blood sugar.

                      Pineal gland: Believed to regulate biorhythms and moods and stimulate the onset of puberty.

                      Pituitary Gland: Regulates other endocrine glands secretes growth hormone.

                      Receptor: A specific site on a cell designed to recognize and accept a specific hormone.

                      Testis: Controls development of secondary sex characteristics and functioning of sex organs.

                      Thymus: Assists in development of the immune system.

                      Thyroid: Regulates metabolic rate.


                      Discussion Topic: Talk with students about the importance of good communication. Discuss what happens when we have problems communicating in the classroom, or with our friends. Talk about why communication is important for us, for astronauts, and for engineers!

                      Voting: Ask a true/false question and have students vote by holding thumbs up for true and thumbs down for false. Count the votes and write the totals on the board. Be sure to tell students the right answer after they vote.

                      • True or False: Engineers do not need to be good communicators. (Answer: False, communication is a very important part of being an engineer.)
                      • True or False: The endocrine system in our bodies is like the mail system, and hormones are like letters that get delivered by our bloodstream. (Answer: True)
                      • True or False: Hormones go to a specific place in our body and tell our body what to do. (Answer: True)

                      Lesson Summary Assessment

                      Matching: Create a list of parts of the endocrine system, and parts of the mail system. Randomly write the endocrine system parts on the left side of the board and the mail system parts on the right side of the board. As a class, have the students match the correct sides together. For example:

                      Bloodstream Mail carrier, who carries the message or letter to the right spot

                      Hormone The message or letter, which has specific instructions in it

                      Organ or Receptor The mailbox, where the message needs to go in exactly the right box!

                      Endocrine gland The person who wrote the letter or is mailing out the instructions

                      (Note: these pairs are sorted correctly, but should be randomly mixed for the students).

                      Lesson Extension Activities

                      Have students research the production of insulin or human growth hormone.

                      Help students research and give a presentation on endocrine disruptors.

                      For older students, teachers may want to discuss the role of illegal steroids in sports.


                      Fox, Stuart Ira. Human Physiology. Seventh Edition. New York, NY: McGraw Hill, 2002.

                      What is Nervous System

                      Nervous system refers to the network of nerve cells that coordinate the functions of the body. The nerve cell is the structural and functional unit of the nervous system. The neuroglia are the supporting cells of the nervous system. It comprises the brain spinal cord, nerves, ganglia, receptor organs, and the effector organs. The parts of the nervous system can be divided into two: central nervous system (CNS) and peripheral nervous system (PNS). The CNS comprises the brain and the spinal cord. The PNS comprises the rest of the peripheral nerves which receive stimuli through sensory neurons and transmit impulses to the effector organs through motor neurons. The peripheral nerves connect the body to the CNS. The PNS receive stimuli from both internal and external environment of the body.

                      Figure 2: Nervous System in Humans

                      The PNS can be further divided into two sections known as somatic nervous system and autonomic nervous system. The somatic nervous system coordinates the voluntary functions that can be regulated by the person. In contrast, the autonomic nervous system coordinates the involuntary functions. Neurotransmitters help the transmission of signals between two nerve cells. The human nervous system is shown in figure 2.

                      Anatomy and physiology of ageing 7: the endocrine system

                      Glands in the endocrine system produce a range of hormones that regulate our body’s activities by keeping substances such as blood glucose and electrolytes within their normal ranges. Like all other body systems, the endocrine system undergoes age-related changes that negatively affect its functioning. As a result of these changes, older people are more prone to disturbed sleep patterns, have a reduced metabolic rate, lose bone density, accumulate body fat, and show increases in blood glucose. As a consequence, they are at higher risk of health issues such as insomnia, fractures, type 2 diabetes and cognitive decline. This seventh article in our series about the effects of age on the body describes what happens, with advancing age, to endocrine glands and hormone production.

                      Citation: Knight J, Nigam Y (2017) Anatomy and physiology of ageing 7: the endocrine system. Nursing Times [online] 113: 8, 48-51.

                      Authors: John Knight is senior lecturer in biomedical science Yamni Nigam is associate professor in biomedical science both at the College of Human Health and Science, Swansea University.

                      • This article has been double-blind peer reviewed
                      • Scroll down to read the article or download a print-friendly PDF here to see other articles in this series


                      The endocrine system works in conjunction with the nervous system to regulate, and coordinate the activities of, the body’s tissues and organs. It consists of a collection of glands located in different parts of the body – the main ones being the pituitary, pineal, thyroid, parathyroids, adrenals, pancreas, ovaries and testes (Fig 1). These glands produce a variety of blood-borne chemical signals called hormones, which play an essential role in maintaining balance (homoeostasis) in the body, helping to ensure that variables such as blood glucose and electrolytes are kept within normal ranges.

                      Pituitary gland and somatopause

                      The pituitary gland, often referred to as the master gland, produces several major hormones and regulates the activity of many other endocrine glands. It is split into a posterior portion, which is formed from neural tissue extending from the hypothalamus, and an anterior portion, which is formed from epithelial cells derived from the roof of the oral cavity.

                      The anterior pituitary secretes growth hormone (somatotropin), which promotes the growth of bone, muscle and most of the major internal organs. In early childhood, somatotropin is secreted in relatively small amounts, but during the teenage years there is a marked increase in serum somatotropin levels corresponding to the growth spurts of puberty. Around the age of 25-30, somatotropin secretion begins to decline in both men and women. In men it is estimated to halve every seven years – although there appears to be much variation between individuals (Gentili, 2015).

                      The decline in somatotropin secretion in later years is often referred to as the somatopause and is associated with a variety of physiological changes (Jonas et al, 2015 Veldhuis et al, 2005), including:

                      • A general reduction in protein synthesis
                      • A progressive reduction in lean body mass (muscle) contributing to a decline in metabolic rate
                      • An increased deposition of adipose tissue, particularly abdominal fat (‘middle-age spread’)
                      • A reduction in bone mass and density leading to an increased risk of osteoporosis and fractures
                      • A general decrease in immune function and higher susceptibility to infection.

                      The somatopause can be hastened in people who lead a sedentary lifestyle and in those who already carry a high percentage of body fat. Conversely, in pre-menopausal women, oestrogen appears to slow its onset and progression (Gentili, 2015).

                      The exact causes of somatopause are yet to be fully established, however, the age-related decrease in somatotropin secretion mirrors the decrease of growth-hormone releasing hormone (GHRH) secretion by the hypothalamus. Recent research indicates that some of the negative physiological changes that come with declining levels of somatotropin can be reversed by growth hormone replacement therapy. In clinical trials, recombinant human growth hormone has been shown to improve lean muscle mass retention and quality of life scores in older people (Jonas et al, 2015).

                      Pineal gland and sleep disturbances

                      The pineal gland is slightly smaller than a pea and resembles a small pine cone – hence its name. Found in the diencephalon, towards the centre of the brain, it synthesises the hormone melatonin from the neurotransmitter serotonin. The pineal gland functions like an internal body clock: during the day, when there is a lot of light, melatonin secretion is inhibited, but as the day draws to a close and light diminishes, melatonin secretion increases, preparing the body for sleep.

                      As we age, the pineal gland undergoes a process of calcification, detectable even in young children. Melatonin levels progressively decrease: 60-year-olds have 80% less melatonin in their blood than teenagers. Some drugs commonly prescribed to older people, such as beta blockers and non-steroidal anti-inflammatory drugs, can reduce melatonin levels even further.

                      Decreased melatonin levels are linked to an increased prevalence of sleep disturbances and, in some people, may ultimately lead to geriatric insomnia (Bubenik and Konturek, 2011). Since sleep is essential for cognitive function, sleep disturbances can exacerbate age-related changes in the brain.

                      There is some evidence that exposure to bright light – either sunlight or artificial light – in the morning increases the speed of sleep onset by triggering an earlier release of melatonin in the evening. Similarly, the therapeutic use of prolonged-release melatonin has been shown to improve sleep onset time, sleep quality, morning alertness and quality of life in people aged 55 and over who have insomnia (Wade et al, 2007).

                      Thyroid gland and metabolism

                      The thyroid gland plays a major role in controlling metabolism and adjusting blood calcium levels. The hormones it secretes regulate a number of physiological processes, including:

                      • The metabolism of carbohydrates, fats and proteins
                      • Thermoregulation
                      • Digestion
                      • Muscle and nerve activity
                      • Maintaining skin thickness and integrity
                      • Maintaining normal bone density.

                      Changes to metabolic rate

                      The thyroid secretes the iodine-containing hormones T4 (tetraiodothyronine, which is also known as thyroxine) and T3 (triiodothyronine), which largely control cellular metabolism. T4 is released in greater quantities than T3, the typical ratio being 15:1. T4 is then rapidly converted into the more biologically active T3, which is around three times more potent in terms of increasing the metabolic rate.

                      The clearance of T4 by the liver decreases with age, but this is offset by a gradual decline in T4 secretion, so T4 serum levels tend to remain constant. However, there is a clear age-related decrease in the levels of serum T3, as well as of thyroid-stimulating hormone (TSH) produced by the pituitary gland (Peeters, 2008 Chahal and Drake, 2007). This may contribute to the gradual reduction in basal metabolism that is apparent in many people in middle and old age (in which the decline in lean muscle mass described above also plays a role).

                      With advancing age, autoimmune reactions against one’s own thyroid gland are commonly seen. Indeed, the presence, in older people, of antibodies specific to thyroid tissue is so common that it is often considered a normal age-related change. A high concentration of such antibodies may herald the onset of autoimmune hypothyroidism, a disease affecting up to 5% of the over-60s and associated with low metabolic rates, a tendency to put on weight and low core temperature. Since this condition is autoimmune in nature, women are at greater risk of developing it (this is true for most autoimmune diseases): up to eight times more women than men experience autoimmune hypothyroidism.

                      The results of thyroid function tests should be assessed carefully in older people, as common long-term conditions (such as chronic obstructive pulmonary disease, hypertension, diabetes and arthritis) and dieting can lead to reductions in circulating thyroid hormones, particularly the more active T3. This phenomenon of reduced thyroid function in the absence of thyroid disease is referred to as non-thyroidal illness. Similarly, many drugs used to treat long-term conditions in older people (for example, lithium and glucocorticoids) can supress thyroid function or reduce the activity of circulating thyroid hormones, leading to a reduction in metabolic rate (Peeters, 2008).

                      Changes to calcitonin secretion

                      The thyroid gland also plays a role in calcium homoeostasis. When we consume foods rich in calcium, it releases calcitonin, which inhibits the activity of osteoclasts – bone cells that break down bone tissue (bone is a dynamic tissue continually being built and broken down). By inhibiting osteoclast activity, calcitonin indirectly increases bone density.

                      Few studies have examined the effects of ageing on calcitonin production in humans. The most comprehensive study, dating back to 1980, demonstrated an age-related decline in calcitonin production in 50 healthy women aged between 20 and 69 years (Shamonki et al, 1980). This decline may partially explain the reduction in bone mass seen in most women as they grow older. However, a later study has contradicted these findings, showing that although women appear to have lower levels of calcitonin secretion than men, there is no clear age-related decrease in serum calcitonin concentration (Tiegs et al, 1986).

                      Parathyroid glands and hyperparathyroidism

                      The posterior portion of the thyroid is the location of four tiny parathyroid glands, which secrete parathyroid hormone (PTH) whenever blood calcium levels fall. Since a normal concentration of calcium is essential to many physiological processes (including muscle contraction, nerve conduction and blood clotting), the reserves of calcium stored in the skeleton need to be mobilised. PTH triggers the release of calcium from the bones into the blood by indirectly stimulating osteoclasts.

                      Several studies have shown that most people, as they grow older, have significantly increased levels of circulating PTH (Portale et al, 1997). This hyperparathyroidism may well be one of the main causes of the reduction in bone density commonly seen in middle and old age. Recent studies have also shown a potential link with other pathologies, particularly age-related cognitive decline and dementia (Braverman et al, 2009).

                      Pancreas and diabetes risk

                      The endocrine regions of the pancreas (islets of Langerhans) regulate blood glucose levels. Beta cells in the islets secrete insulin in response to increased blood glucose – for example, after a carbohydrate-rich meal. Insulin binds to receptors present on most cells, triggering the uptake of glucose from the blood. Once inside the cells, glucose is either metabolised immediately to release energy, or stored and converted into glycogen.

                      Alongside race, genetic predisposition and a high body mass index, ageing is one of the many risk factors linked to the development of type 2 diabetes (Knight and Nigam, 2017). Ageing human cells become less sensitive to the effects of insulin. The most likely cause appears to be a reduction in the number of insulin receptors at the surface of cells. This gradual insulin resistance goes hand in hand with an increase in blood glucose concentrations.

                      As shown in a study of 6,901 non-diabetic people (Ko et al, 2006), fasting blood glucose levels rise by around 0.15mmol/L for each decade of life after the age of 20. Whether this rise is a normal age-related change or a sign of diabetes in its early stages is not always clear, but it is certainly seen in many older people with no other symptoms of diabetes.

                      With advancing age, the insulin-producing beta cells become less sensitive to the level of glucose in the blood, so higher blood glucose levels are needed to trigger insulin release. Since older people’s cells are less receptive to insulin, the pancreas often responds by producing more, leading to increased insulin levels in the blood (hyperinsulinaemia). This can put excessive stress on the beta cells, leading to their exhaustion.

                      Age-related depletion of the beta cell population in the pancreas also occurs as a result of increased programmed cell death (apoptosis) and a diminished ability of the pancreas to produce new cells. Beta cell exhaustion and depletion result in a drop of insulin secretion of up to 0.5% per year of life. Additionally, the clearance of insulin by the liver increases with age, so there is less insulin available to interact with cells and promote glucose uptake.

                      These age-related changes to insulin production, clearance and response contribute to the creation of a diabetogenic environment. This may partially explain why the risk of developing type 2 diabetes increases with age (Brown, 2012).

                      Abdominal fat

                      The accumulation of abdominal fat is a common feature of ageing, particularly in people who have a poor diet and/or a sedentary lifestyle. Many age-related changes to the endocrine system contribute to this accumulation of adipose tissue, including the somatopause, autoimmune hypothyroidism, insulin resistance, and reduced circulating sex hormones.

                      This abdominal fat accumulation is linked to heart disease, high blood pressure and type 2 diabetes. These conditions may occur in isolation or together in the form of metabolic syndrome (Gong and Muzumdar, 2012).

                      The adrenal glands

                      The two adrenal glands are located above the kidneys and each consists of two main regions: the adrenal medulla (inner region) and the adrenal cortex (outermost layer).

                      Adrenal medulla and adrenaline

                      The adrenal medulla is the location of chomaffin cells, which secrete the catecholamines adrenaline (epinephrine) and noradrenaline (norepinephrine). These are the ‘fight or flight’ hormones that prepare the body for activity when it is threatened or in a state of excitement. The effects of adrenaline and nor-adrenaline include:

                      • Increased heart rate
                      • Increased vasoconstriction in the skin and gut
                      • Increased blood pressure
                      • Increased blood flow to the major skeletal muscle groups
                      • Increased blood flow to the brain
                      • Dilatation of pupils
                      • Increased breathing rate and airway dilatation
                      • Increased breakdown of liver glycogen resulting in increased blood glucose.

                      Ageing is associated with a decline in the secretion of adrenaline, but adrenaline plasma levels remain relatively constant as clearance by the kidneys is usually reduced. There is some evidence that older men secrete less adrenaline in response to acute stress than younger men (Seals and Esler, 2000).

                      Adrenal cortex, aldosterone and cortisol

                      The adrenal cortex synthesises a variety of steroidal hormones from cholesterol, mainly aldosterone and cortisol.

                      Aldosterone is a mineralocorticoid that regulates plasma levels of sodium and potassium, and plays an important role in water balance and blood pressure control. Research has revealed an age-related decrease in serum aldosterone levels, effectively reducing the body’s ability to retain sodium.

                      Decreased aldosterone secretion may contribute to postural hypotension and the light-headedness that is often experienced by older people when they stand up. This is supported by research demonstrating significant reductions in serum aldosterone levels in older people when they are upright, as opposed to recumbent (Hegstad et al, 1983).

                      Since sodium attracts water into the cardiovascular system via osmosis, lower plasma sodium levels (hyponatraemia) can lead to reduced blood volume and blood pressure. Several medications commonly prescribed to older people – such as opiates, non-steroidal anti-inflammatory drugs, diuretics and antidepressants – can exacerbate hyponatraemia (Liamis et al, 2008). Blood volume and blood pressure may be further reduced by age-related increases in the secretion of atrial natriuretic hormone (ANH), a powerful diuretic produced by the heart (Miller, 2009).

                      Cortisol is a glucocorticoid and its release is triggered by biological stressors such as physical injury or starvation. It is a natural anti-inflammatory and plays an important role in the breakdown of protein and fat.

                      Research into how cortisol levels change with ageing is often contradictory. Initial studies suggested that there could be a 20-50% increase in the mean levels of cortisol secretion between the ages of 20 and 80 (Chahal and Drake, 2007). More recently, however, it has been shown that this is not necessarily true: in some people, cortisol secretion diminishes with age, in others levels remain relatively stable throughout life (Wolf, 2015).

                      There appears to be a link between increased cortisol levels, reduced bone density and increased risk of bone fracture. There is also growing evidence that a higher cortisol concentration can contribute to the loss of cells from the hippocampus, resulting in hippocampal atrophy. This is often associated with a reduction in cognitive function in older people (Chahal and Drake, 2007). Other studies have shown that age-related increases in cortisol may also be linked to memory loss and sleep disorders (Chahal and Drake, 2007 Wolf et al, 2005).

                      Ageing of the endocrine system

                      There is some evidence that exercising regularly and maintaining a low percentage of body fat may slow the onset of the somatopause, help maintain bone density and improve the control of blood glucose. Supplementation with synthetic growth hormone has recently been shown to increase lean muscle mass in older people. However, this kind of therapy is associated with many side-effects such as joint pain, oedema and impaired glucose tolerance (Jonas et al, 2015).

                      The most famous and most thoroughly researched hormone replacement therapies are those that are used to treat the complications of the menopause. These therapies will be explored in the next article in this series.

                      Key points

                      • The endocrine system regulates our physiology via a complex cascade of chemicals called hormones
                      • Hormones play an essential role in maintaining homoeostasis in the body
                      • Age-related hormonal changes can lead to the build-up of body fat, lower bone density, impaired blood glucose control and sleep disturbances
                      • Age-related changes to the endocrine system increase the risk of insomnia, fractures, type 2 diabetes and cognitive changes
                      • Exercising regularly and maintaining a low percentage of body fat may slow down some of the effects of ageing on the endocrine system

                      Also in this series

                      Braverman ER et al (2009) Age-related increases in parathyroid hormone may be antecedent to both osteoporosis and dementia. BioMed Central Endocrine Disorders 9: 21, 1-10.

                      Brown JE (2012) The ageing pancreas. British Journal of Diabetes and Vascular Disease 12: 3, 141-145.

                      Bubenik GA, Konturek SJ (2011) Melatonin and aging: prospects for human treatment Journal of Physiology and Pharmacology 62: 1, 13-19.

                      Chahal HS, Drake WM (2007) The endocrine system and ageing. Journal of Pathology 211: 2, 173-180.

                      Gong Z, Muzumdar RH (2012) Pancreatic function, type 2 diabetes, and metabolism in aging. International Journal of Endocrinology 2012: 320482.

                      Hegstad R et al (1983) Ageing and aldosterone. American Journal of Medicine 74: 3, 442-448.

                      Jonas M et al (2015) Aging and the endocrine system. Postępy Nauk Medycznych 28: 7, 451-457.

                      Knight J, Nigam Y (2017) Diabetes management 1: disease types, symptoms and diagnosis. Nursing Times 113: 4, 40-44.

                      Ko GT et al (2006) Effects of age on plasma glucose levels in non-diabetic Hong Kong Chinese. Croatian Medical Journal 47: 5, 709-713.

                      Liamis G et al (2008) A review of drug-induced hyponatremia. American Journal of Kidney Disease 52: 1, 144-153.

                      Miller M (2009) Fluid balance disorders in the elderly. American Society of Nephrology online curricula: geriatric nephrology.

                      Peeters RP (2008) Thyroid hormones and aging. Hormones 7: 1, 28-35.

                      Portale AA et al (1997) Aging alters calcium regulation of serum concentration of parathyroid hormone in healthy men. American Journal of Physiology 272: 139-146.

                      Seals DR, Esler MD (2000) Human ageing and the sympathoadrenal system. Journal of Physiology 528: 3, 407-417.

                      Shamonki IM et al (1980) Age-related changes of calcitonin secretion in females. Journal of Clinical Endocrinology and Metabolism 50: 3, 437-439.

                      Tiegs RD et al (1986) Secretion and metabolism of monomeric human calcitonin: effects of age, sex, and thyroid damage. Journal of Bone and Mineral Research 4: 339-349.

                      Veldhuis JD et al (2005) Joint mechanisms of impaired growth-hormone pulse renewal in aging men. Journal of Clinical Endocrinology and Metabolism 9: 7, 4177-4183.

                      Wade AG et al (2007) Efficacy of prolonged release melatonin in insomnia patients aged 55-80 years: quality of sleep and next-day alertness outcomes. Current Medical Research and Opinion 23: 10, 2597-2605.

                      Wolf OT (2015) Effects of Stress on Memory: Relevance for Human Aging. Encyclopedia of Geropsychology. Singapore: Springer Science.

                      Wolf OT et al (2005) Subjective memory complaints in aging are associated with elevated cortisol levels. Neurobiology of Aging 26: 10, 1357-1363.

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                      An Overview of the Adrenal Glands

                      The adrenal glands are two glands that sit on top of your kidneys that are made up of two distinct parts.

                      • The adrenal cortex—the outer part of the gland—produces hormones that are vital to life, such as cortisol (which helps regulate metabolism and helps your body respond to stress) and aldosterone (which helps control blood pressure).
                      • The adrenal medulla—the inner part of the gland—produces nonessential (that is, you don’t need them to live) hormones, such as adrenaline (which helps your body react to stress).
                      When you think of the adrenal glands (also known as suprarenal glands), stress might come to mind. And rightly so—the adrenal glands are arguably best known for secreting the hormone adrenaline, which rapidly prepares your body to spring into action in a stressful situation.