Endocrine System
Glands And Hormones: Function And Role In The Body

Health Topics


Endocrine System


What Is The Endocrine System?
The Glands And The Hormones They Produce
What Is The Role Of The Sex Hormones?
What Can Go Wrong With The Endocrine System?
Other Systems It Links To

Related Articles

The Female Body: Diagrams
The Human Body: Organs and function.
Infertility Guide: Fertility and hormones.



What Is The Endocrine System?

The endocrine system is one of the body's communication systems. It uses hormones to tell the body what to do. The endocrine system is composed of ductless glands. These are glands that release secretions directly into the blood and not into a duct (tube) like other types of glands such as the salivary glands. These ductless glands produce hormones, the body's chemical messengers. Each gland produces specific hormones. Hormones control and affect many body functions and organs, as well as behavior. The function of the endocrine system is closely linked to that of the nervous system.


What Is A Hormone?

A hormone is a chemical messenger. It is secreted directly into the blood by one of the glands in the endocrine system. When they are released they trigger reactions in different parts of the body. Some hormones are made of protein (insulin for example), while others are steroids (adreno-corticoid hormones), glycoproteins (TSH, FSH and LH) or derivatives of single amino acids (T4,T3). Hormones are produced in the gland and are then transported to the part of the body (or organ) they control or affect.

What Is An Endocrine Gland?

A ductless gland which produces hormones. Ductless means that there is no separate canal or tube to transport the hormones to the blood. Hormones travel straight into the bloodstream from the gland. Most hormones in the body are produced by the endocrine glands.

What Do Hormones And The Endocrine System Do?

They cause behaviors and functions in different areas of the body - for example some hormones are responsible for correct growth in children and changes to the body in puberty, others control the menstrual cycle, pregnancy or cause menopause. Some are responsible for responses to stress and danger and the proper functioning of the kidneys and digestive system.


The following section explains the location of each gland and which hormones they produce. It also explains what each hormone does and what can go wrong with it. If too much of a hormone is produced it is known as hypersecretion; too little is known as hyposecretion.

Pituitary Gland
Situated at the base of the brain, sitting in a cup-shaped depression of the skull called the sella turcica. It is closely connected to the hypothalamus. The pituitary gland has 2 hormone-secreting lobes, the anterior and posterior.

Anterior Lobe Hormones

Hormone: Human growth hormone (HGH)
Function: It regulates height and growth. Along with genes it is the main controller of final height of a person.
Malfunctions: Hypersecretion causes gigantism (in children) or acromegaly (in adults). Hyposecretion causes dwarfism.

: Melanocyte-stimulating hormone (MSH)
Function: Stimulates production of the brown pigment melanin in the skin.

Hormone: Thyrotrophin (TSH)
Function: Controls the thyroid gland.
Malfunctions: Go to thyroid gland.

Hormone: Adrenocorticotrophin (ACTH)
Function: Controls the adrenal cortex.
Malfunction: Go to adrenal cortex.

Hormone: Prolactin or lactogenic hormone (LTH)
Function: Production of milk during lactation.

Hormone: Gonadotrophins (gonad/sex organ hormones)
Function: Controls sexual development and organs (ovaries and testes)

Hormone: Follicle-stimulating hormone (FSH)
Function: Stimulates ovaries to produce estrogen and to ovulate (release an egg for fertilization).

Hormone: Luteinizing hormone (LH), also known as lutropin or lutrophin
Function: Stimulates ovaries to produce the corpus luteum (for ovulation) from ruptured follicle.

Hormone: Interstitial cell-stimulating hormone (ICSH)/luteinizing hormone in men
Function: Stimulates sperm production and secretion of testosterone.
Malfunctions (of gonadotrophin): Polycystic ovarian syndrome, endometriosis and uterine fibroids.

Posterior Lobe Hormones

Hormone: Antidiuretic hormone (ADH or vasopressin)
Function: Regulates amount of water absorbed in the kidneys.
Malfunctions: Hyposecretion causes diabetes insipidus and hypersecretion: edema (swelling).

Hormone: Oxytocin (the love hormone)
Function: Studies show that raised levels of oxytocin help human bonding and trust (hence the term love hormone). It simulates the secretion of breastmilk for feeding and tells the muscles of uterus to contract for childbirth.
Thyroid Gland
Location: The thyroid gland is shaped like a bow-tie and is positioned at the front of the neck, just below the larynx (voice box).

Hormones: Thyroxin, and triiodothyronine (produced in response to TSH from anterior lobe of Pituitary Gland).
Functions: Stimulate tissue metabolism and maintains BMR (basic metabolic rate).
Malfunctions: Hypersecretion known as Graves disease or thyrotoxicosis; hyperthyroidism (increase in metabolic rate, heart rate, anxiety, intolerance of heat plus raised temperature, frequent bowel action). Hyposecretion: causes the body systems to slow below normal speed (hypothyroidism), cretinism (at birth) or myxedema (disorder caused later in life by untreated cretinism), goiters (enlarged thyroid).

Hormone: Calcitonin
Function: Maintains calcium and phosphorus balance in the body.
Malfunction: Hypersecretion causes lowering of blood calcium level by preventing loss of calcium from bone. Is this good or bad? Scientists don't really know. To date they can find no adverse reaction when levels are high or low.

Parathyroid Glands
Location: There are 4, two either side behind the thyroid gland.
Hormone: Parathormone
Functions: Maintains calcium level in blood plasma. Stimulates calcium reabsorption by the kidneys and activates Vitamin D.
Malfunctions: Hypersecretion causes hyperparathyroidism (symptoms include depression, bone pain, feeling tired and kidney stones). It also softens the bones leading to spontaneous bone fractures. Hyposecretion causes hypoparathyroidism: abnormally low blood calcium levels; tetany (spasms in hands and feet caused by over-contraction in muscles) and convulsions (from over-stimulated nerves).
Adrenal Glands
Location: There is one sitting on top of each kidney. Each gland is split into 2 parts: the adrenal cortex and the adrenal medulla.

Adrenal Cortex

Hormones: Mineralocorticoids - aldosterone (steroids)
Function: Regulates salts in body, especially sodium chloride and potassium.
Malfunctions: Hypersecretion causes kidney failure, high blood pressure, too much potassium in blood causing abnormal heart beat (heart arrhythmia). Hyposecretion can cause Addison's disease, muscular atrophy and weakness; and body systems slow down.

Hormones: Glucocorticoids (steroids) (cortisol and cortisone)
Functions: Produced in response to ACTH (from pituitary gland). It metabolizes carbohydrates, fats and proteins.
Malfunctions: Stunted growth. Hypersecretion causes Cushing's syndrome, hypertension, moon-shaped face, muscular atrophy and diabetes.

Hormones: Female sex hormones (steroids): estrogen and progesterone (men have small levels). Male sex hormones: testosterone (small amounts also secreted in the ovaries in women).
Functions: Help the body sexually develop and mature; ovulation; hair growth in pubic and under arm areas.
Malfunctions: Lots of problems! Including hirsutism, amenorrhea (hypersecretion of testosterone in women); muscle atrophy and breast growth (hypersecretion of estrogen in men). Hyposecretion can cause Addison's disease.

Adrenal Medulla
The adrenal medulla functions to support the sympathetic nervous system.
Hormones: Adrenaline and noradrenaline
Function: Usually called the stress hormones, they prepare the body for 'fight or flight' by speeding up heart rate, slowing digestive system and urinary system, increasing blood pressure and blood sugar level. Adrenaline is a powerful vasoconstrictor - that means it constricts blood vessels in order to increase blood pressure.

Specifically in the islets of Langerhans, specialized cells that form part of the pancreas.
Location: Slightly below and behind the stomach.
Hormones: Insulin and glucagon
Function: Tells the cells to open and accept glucose in, thus regulating blood sugar levels.
Malfunctions: Hyposecretion: diabetes (high blood sugar level and high urine production); symptoms of diabetes include fatigue; weight loss; coma. Hypersecretion can lead to hypoglycemia (low blood sugar level) including symptoms of hunger and sweating; in serious cases may lead to coma.

Location: There are 2 ovaries, one on each side of the womb (uterus).
Hormones: Estrogen and progesterone (female sex hormones, although male testes also produce small amounts)
Functions: Responsible for female sexual characteristics - breast growth, widening of hips, pubic and underarm hair.
Malfunctions: Cause unknown. In women can lead to polycystic ovarian syndrome (also known as Stein-Leventhal syndrome). In men it can lead to muscle atrophy and breast growth.
Location: Inside the scrotum behind the penis.
Hormone: Testosterone (male sex hormone, although ovaries produce small amounts in women).
Functions: Responsible for male sexual characteristics thus sperm production, changes at puberty — voice breaking, pubic, facial and underarm hair growth, increased muscle mass.
Malfunctions: Low levels of testosterone is normal in women. Hypersecretion on the other hand can lead to virilism, hirsutism and missed periods.
Pineal Gland
Location: Also called the pineal body. It is located at the center of the brain.
Hormone: Melatonin (derived from serotonin)
Function: Controls body rhythms (our internal clock, tells us when to sleep and wake up). It responds to sunlight.
Malfunctions: Jet-lagged feeling; depression and seasonal affective disorder (SAD).
Thymus Gland
Location: In the upper part of the chest (thorax).
Hormone: Thymic Factor (TF), thymic humoral factor (THF), thymosine, thymopoietin
Functions: Part of immune system, appears to promote development of T Lymphocytes in the thymus gland. These fight infections in the body.
Malfunction: Lowered resistance to infections and/or stress.

What Is The Role Of The Sex Hormones?

Puberty is the age at which the internal reproductive organs of boys and girls reach maturity and become functional. Although the effect on these organs cannot be seen, the effect on the rest of the body can, in the form of secondary sexual characteristics. Puberty starts about the age of 9 or 10 for girls (between 12 and 14 for boys) and lasts roughly 5 years. See, the development of the female body to read more about puberty.

Menstrual Cycle
One of the most important functions of hormones is to prepare the body for reproduction. In a male this involves sperm production. In a female, it involves producing ova (eggs) and preparing the womb so that a fertilized egg can grow into a baby. Whether an egg is fertilized or not, the process of preparing a woman's body for having a baby happens every month. This is known as the menstrual cycle. The start of menstruation (for the first time) is called the menarche. Every 28 days from puberty to menopause (approximately 35 years) the body will prepare itself for a baby and if fertilisation does not take place the body will undo its preparations before starting again a few weeks later.

There are 3 stages:
- First menstrual phase
- Second proliferative phase
- Third secretory phase.

First Menstrual Phase
This lasts for approximately 5 days. Progesterone produced by the corpus luteum enters the bloodstream and the pituitary gland responds by producing less luteinizing hormone. But less luteinizing hormone means the corpus luteum begins to break down, the progesterone level falls which stops the endometrium from holding fluid and it starts to breakdown as well. Menstruation begins as a result of the breakdown of the endometrium (the lining of the womb). Menstrual flow contains:
- Mucus
- Cells from the lining of the womb
- Blood from broken capillaries in the endometrium
- Unfertilized egg.

Second Proliferative Phase
This lasts for approximately 7 days. Follicle-stimulating hormone (FSH) is produced in the anterior lobe of the pituitary gland and this stimulates the follicles of the ovaries to produce estrogen. These follicles are small structures on the surface of the ovary. The estrogen then stimulates the endometrium, promoting the growth of new blood vessels and mucus-producing cells (hence the name proliferative, meaning reproduction and growth). At the end of this stage ovulation occurs: a mature Graafian follicle ruptures releasing a single egg which then travels along the Fallopian tube to the uterus.

The Secretory Phase
This lasts for approximately 14 days. Luteinizing hormone (LH), secreted in the anterior lobe of the pituitary gland, stimulates the ruptured follicle lining to grow into corpus luteum - a temporary structure formed by the effect of this hormone on the ruptured ovarian follicle. The corpus luteum produces progesterone thus stimulating the endometrium to retain fluid and produce mucus, which makes it easier for sperm to pass through the reproductive tract. After ovulation, the egg can only be fertilized during the next 12 to 48 hours. If it is not fertilized, the first menstrual phase begins. As soon as progesterone levels drop (a result of the collapse of the corpus luteum and endometrium), the pituitary gland starts the production of FSH again, and thus stimulates the ovaries to produce another follicle and then another ovum. The cycle begins again.

When Does My Menstrual Cycle Stop Happening?
Once the menarche (start of menstruation) has passed, menstruation only stops in 3 instances:
- Amenorrhea
- Pregnancy
- Menopause

A woman can, technically speaking, bear children as long as she is menstruating. This reproductive period lasts about 35 years, until the ova (egg) supply is exhausted. Women are born with a certain number of eggs. When these run out, the menopause begins. The average age for menopause to begin is 45 to 55 and it takes an average of 5 years to complete (though it can last 10). During this period the hormonal changes that began with puberty will be reversed. For example the ovaries will gradually stop responding to FSH and LH, the hormones that provoked changes in puberty, and thus produce less estrogen and progesterone. The reduction in these hormones causes irregular menstrual cycles (before menstruation stops completely), shrunken breasts, less hair growth on the body, hot flashes, night sweats, palpitations and depression. Many of these symptoms of menopause can be alleviated by use of Hormone Replacement Therapy (HRT).

What Can Go Wrong With The Endocrine System?

Addison's Syndrome
Cause: Hyposecretion (too little) of adrenocortical hormones (sex, growth and salt regulation hormones).
Effects: Muscle waste (atrophy) and weakness; high blood pressure; gastric problems like vomiting, changes in skin pigmentation, irregular menstrual cycle and dehydration.

Cause: Linked to high levels of testosterone (in women), stress; radical weight loss and anemia.
Effect: Missed periods.

Cushing's Syndrome
Cause: Excess levels of adrenocortical hormones (sex, growth and salt regulation hormones). It is the opposite of Addison's syndrome.
Effects: Muscular atrophy and weakness, high blood pressure, moon-shaped face, redistribution of body fat, sometimes mental illness and osteoporosis (brittle bones).

Premenstrual Syndrome (PMS)
Cause: The start of menstruation - usually starts about one week before.
Effects: PMS can cause depression, bloating, irritability, swollen and tender breasts, restlessness.

Polycystic Ovary Syndrome (PCOS)
Cause: Still not known.
Effects: The symptoms of PCOS include irregular periods, follicular ovarian cysts and sometimes infertility, enlarged ovaries and often high levels of estrogen. 50 percent of PCOS patients are obese and and suffer hirsutism (excess body or facial hair).

Cause: Endometriosis is a condition where fragments of the womb end up in other parts of the body instead of being shed during a monthly menstrual bleed. The doctors are not sure what causes endometriosis, but it may be linked to excessive estrogen levels.
Effects: Symptoms include painful periods, female infertility, frequent yeast infections and exhaustion.

Ovarian Cysts
Cause: An ovarian cyst is an abnormal sac filled with fluid, rather like a blister, that forms on or inside the ovary. The cause is not yet known.
Effects: Irregular or missed periods, cramps on one side of the body, spotting between periods and tender breasts. See, signs of ovarian cysts.

Stress is a threat to the body and the body responds to it like any other danger - the adrenal medulla releases adrenaline and noradrenaline to help us with the fight or flight response. The physical manifestations of the arrival of adrenaline in the body are faster heart rate and breathing, sweating (hence sweaty palms when we are frightened or nervous), a glucose rush from the liver and heightened senses (like hearing and sight). Prolonged stress may cause amenorrhea in women and low production of sperm in men. Read more about the dangers of stress.

Type 1 Diabetes
Cause: Type 1 diabetes is an autoimmune disease where the pancreas cannot produce insulin.
Effects: Symptoms include high levels of blood glucose, excessive thirst, peeing frequently, tiredness and weight loss. There is no cure but it can be controlled by regular (2 to 4 per day) insulin injections.

Type 2 Diabetes
Cause: With type 2 diabetes the body cells do not respond properly to insulin and the pancreas may not produce enough. Closely linked to obesity.
Effects: Symptoms are as Type 1, but may not be as obvious and take longer to develop. Prediabetes is an early warning. Type 2 diabetes can be controlled by a healthy diet and physical activity. Tablets or insulin injections may also be required.

A goiter (also spelled goitre) is an abnormally enlarged thyroid gland; can result from underproduction or overproduction of hormone or from a deficiency of iodine in the diet.

Grave's Disease
Graves disease is an auto-immune disease. It most commonly affects the thyroid, causing it to grow to twice its size or more, be overactive, with related hyperthyroid symptoms such as increased heartbeat, muscle weakness, disturbed sleep and irritability.

Hyperthyroidism (also called (thyrotoxicosis) is an overactive thyroid gland. It is a glandular disorder resulting from an overproduction of thyroid hormones.

Hypothyroidism is an underactive thyroid gland; a glandular disorder resulting from insufficient production of thyroid hormones.

The inability to sleep - also referred to as chronic sleeplessness.

The bones of the hands, face and feet are enlarged. It is often accompanied by muscle pain, headaches and emotional problems. It is caused by the overproduction of growth hormones in the pituitary gland due to a tumor (in adults).

Abnormally large growth in a young person, usually caused by excessive secretion of growth hormone from the pituitary gland.

High levels of parathyroid hormone are secreted which results in abnormally high levels of calcium in the blood. It can affect many systems of the body (especially causing bone resorption and osteoporosis).

Not enough parathyroid hormone being secreted resulting in abnormally low levels of calcium in the blood.

Summary Of The Endocrine System

• It consists of ductless glands.
• It produces hormones which affect behavior and function.
• It plays a major role in growth, puberty, the reproductive cycle (menstruation, production of sperm, pregnancy, menopause), responses to stress, kidney and digestive functions.

What Systems It Links To

- Nervous System: Works very closely with the nervous system to provide homeostasis - balance in the body. The pituitary gland (endocrine) has an infinite link to the hypothalamus (nervous system/brain) both of which exert great control over the body.
- Circulatory System: Hormones are secreted and carried in the bloodstream to the various target organs.
- Digestive System: Digestion is reliant upon hormones secreted in the stomach, small intestines and pancreas.
- Female Reproductive System: Governs the reproductive system particularly in females as it controls the menstrual cycle and the release of hormones during pregnancy and childbirth.


Other Useful Guides
Respiratory system: Breathing, how we breathe, the lungs and what can go wrong.
Skin structure and function: In depth look at the skin and its role.
Lymphatic system: The body's drainage system.
Bones Of The Body: How they work and what they do.
How menopause affects the body: What is going on inside your body? Find out...

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