What Is the Endocrine System?
The endocrine system consists of multiple organs and glands located throughout the body. These glands produce hormones that regulate most body systems, including metabolism, emotions, fertility, and heart rate.
This article outlines the anatomy and functions of the endocrine system. It also includes information about how common endocrine disorders are diagnosed and treated.
Your endocrine system is made up of three main parts:
- Glands: Small organs that produce and release hormones
- Hormones: Chemical messengers that travel through the bloodstream to send messages to tissues or organs
- Cell receptors: The targets on cells that receive hormone signals
The endocrine system is partially controlled by a pea-sized organ called the hypothalamus, which acts as a bridge between the nervous system and the pituitary gland.
When the hypothalamus receives a signal from the nervous system, it releases hormones that tell the pituitary gland what hormones to produce and/or release.
The pituitary gland releases hormones into the bloodstream that travel to their target cells to activate or inhibit them.
The endocrine system keeps the body’s systems stable. The hypothalamus closely monitors when there is too much or too little of a hormone’s activity, and responds by telling the pituitary gland to increase or decrease hormone production and release.
Receptors and hormones are very specific. Only one type of hormone will fit in its specific receptors on the cell.
Endocrine Glands and Hormones
There are eight major endocrine glands in the body, along with numerous minor ones. When bound to a hormone from the pituitary gland, endocrine glands produce hormones of their own that perform specific functions. The major endocrine glands are as follows:
The pituitary gland has two lobes: the anterior lobe and the posterior lobe. The anterior lobe receives signals from the hypothalamus to produce hormones. The posterior lobe does not make its own hormones—it secretes two hormones produced by the hypothalamus.
- Prolactin: The hormone that stimulates glands in the breasts to grow and produce milk during and after pregnancy
- Somatropin: The growth hormone that stimulates bone and tissue growth throughout the body
- Follicle stimulating hormone (FSH): Stimulates sperm production in males, and helps regulate the menstrual cycle and egg growth in females
- Luteinizing hormone (LH): A hormone that stimulates the release of sex hormones—estrogen and progesterone for females and testosterone for males
- Thyrotropin: The hormone that stimulates the thyroid to release hormones that are responsible for such processes as your body’s development and metabolism
- Adrenocorticotropin hormone: The hormone that stimulates the adrenal glands to produce cortisol, which helps regulate your metabolism, immune system, stress response, and more
- Anti-diuretic hormone (ADH): The hormone that tells your kidneys how much water to filter out of your blood and into your urine
- Oxytocin: A hormone responsible for social bonding, sexual pleasure, releasing breast milk, and more
The thymus is used primarily in childhood, as its role is to secrete hormones that help the immune system develop.
Around the time of puberty, its tissues become replaced with fat, at which point the thymus is no longer necessary for normal immune function.
Hormones secreted by the thymus include:
- Thymosin: The hormone that stimulates production of T cells—white blood cells that help your body fight viruses, bacteria, and cancer
- Thymopoietin: The youth hormone that influences how fast your skin ages and prevents your skin and brain cells from aging too fast
- Thymulin: Another hormone that is essential for development in youth and T cell function
Located within the brain, the pineal gland is a small gland that secretes melatonin—a hormone that regulates the sleep-wake cycle. Melatonin is also important for your immune system, and it helps reduce inflammation in the body.
The thyroid is a gland found on the windpipe in the front of the throat. It uses iodine from foods to produce three hormones:
- Thyroxine (T4): A hormone that plays a role in your metabolism, mood, and body temperature
- Tri-iodothyronine (T3): A hormone that helps regulate metabolism
- Calcitonin: A hormone that helps regulate calcium levels
The thyroid also has four tiny parathyroid glands. They produce parathyroid hormone, which controls levels of calcium and phosphorus in the body.
There are two adrenal glands—one located on top of each kidney. Each adrenal gland is divided into two regions, the cortex and medulla, which have very different functions.
The hormones produced by the adrenal cortex include:
- Glucocorticoids: A group of hormones that fight inflammation in the body and are essential to metabolism, circulation, mood, and the sleep-wake cycle
- Mineralocorticoids: A group of hormones that maintain the balance of water, salt, and potassium in the bloodstream
- Androgens and estrogen: A portion of androgens and small amounts of estrogen are produced in the adrenal cortex
Hormones produced by the adrenal medulla include:
- Epinephrine: AKA adrenaline, the hormone that is released when your fight-or-flight response is activated
- Norepinephrine: Along with epinephrine, norepinephrine raises heart rate and blood pressure and increases blood sugar (glucose) during the fight-or-flight response
The pancreas is a large gland in the abdomen that secretes two hormones, both of which are essential for maintaining normal blood sugar (glucose) levels:
- Glucagon: The hormone that stimulates the liver to release more glucose into the body
- Insulin: The hormone that helps your cells take up glucose and turn it into energy
In females, these two small glands produce three hormones necessary for sex and reproduction:
- Estrogen: The female sex hormone that regulates the menstrual cycle, causes breasts and pubic hair to grow, helps maintain strong bones, and more
- Progesterone: A hormone that helps regulate the menstrual cycle and prepares the uterus for pregnancy when an egg is fertilized by sperm
- Inhibin: A hormone that controls levels of follicle-stimulating hormone, which regulates egg development
A pair of glands found only in males, the testes secrete testosterone—a hormone that regulates male sex drive and sperm production. It’s also responsible for developing and maintaining such male sex characteristics as facial hair and deep voices, along with denser muscle and bone mass.
Types of Endocrine Disorders
Any time one of these hormones is out of balance, many other systems, glands, and hormones can be affected.
Disorders associated with the endocrine system include:
- Polycystic ovary syndrome: A disorder that occurs when altered levels of FSH, LH, androgens, or insulin affect female estrogen levels. The result may include changes in weight, metabolism, and energy.
- Diabetes: A disease in which the pancreas does not produce enough insulin, resulting in high blood sugar levels. Symptoms include frequent urination, fatigue, blurry vision, and extreme hunger.
- Osteoporosis: Low levels of estrogen, often due to menopause, results in bone loss and brittle bones. Osteoporosis can also be caused by a calcium deficiency or high levels of parathyroid hormone.
- Addison’s disease: A condition in which the adrenal glands do not produce enough cortisol or aldosterone. Symptoms include abdominal pain, abnormal menstrual cycles, depression, and salt cravings.
- Hypothyroidism: A condition in which the thyroid does not produce enough thyroid hormones. Symptoms include fatigue, increased sensitivity to cold temperatures, dry skin, and weight gain.
- Hyperthyroidism: A condition in which the thyroid produces too many hormones, leading to weight loss, hand tremors, irregular heartbeat, increased appetite, itchy skin, and more.
- Cushing’s syndrome: Also known as hypercortisolism, Cushing’s syndrome occurs when the adrenal glands produce too much cortisol. This leads to such symptoms as fatigue, depression, muscle weakness, and fertility issues.
Endocrine disorders develop for a number of reasons. An injury, infection, genetic disorder, disease, or tumor can cause a disorder. An endocrine disorder may begin immediately or take years to develop after a gland is injured.
If your primary care doctor suspects your symptoms could be related to your endocrine system, they will likely run blood tests to check for hormone imbalances. You may be asked to give a sample of your urine or saliva.
Depending on the results, you may be referred to an endocrinologist, a doctor who specializes in conditions related to the endocrine system.
Endocrinologists perform more detailed tests to determine the cause of your hormone imbalance. This often includes what are known as stimulation and suppression tests.
For these tests, you will be administered hormones that start (stimulate) or suppress (slow) certain hormones from being produced. The endocrinologist will then assess how your body responds.
For example, to check for Cushing’s syndrome, the doctor would give a steroid called dexamethasone, which suppresses ACTH, thereby suppressing the production of your own cortisol. Then a blood test that measures the body’s cortisol level will help assess whether the adrenal glands are making too much cortisol.
In some cases, a cancerous or non-cancerous growth on a gland may cause the gland to produce too many hormones. To confirm or rule out a tumor, doctors may order a CAT scan or MRI to view the gland in more detail.
Most endocrine disorders cannot be cured, and if untreated, some can become life-threatening. That said, when endocrine disorders are diagnosed and treated early, hormone imbalances and symptoms can be managed.
The first line of treatment is usually hormone therapy. This may take the form of hormone replacement, in which you take a hormone you do not have enough of. Or, if your body is producing too much of a hormone, you may be prescribed hormone suppression therapy to slow the hormone’s production.
Hormone therapy is typically taken in tablet form, or sometimes via injections or a patch worn on the skin. One such example is the estradiol skin patch, which delivers estrogen to relieve symptoms of menopause and may prevent osteoporosis.
Should a tumor be the cause of a hormone imbalance, surgery to remove the tumor is generally indicated.
The endocrine system is a complex network of glands, hormones, and receptors that control most body processes. Its mastermind is the hypothalamus, a tiny organ in your brain with a big purpose: to keep those body processes stable with the right dose of hormones.
Endocrine disorders can develop for many reasons beyond your control. If your doctor is concerned that you could have a hormonal imbalance, they will test your hormone levels and possibly prescribe treatments to stabilize them.
A Word From Verywell
Fatigue, weight changes, and depression are all common symptoms across many endocrine disorders. If you experience these issues, you may be inclined to chalk your symptoms up to a packed schedule or stress.
Nonetheless, you should never feel like your body is out of your control. See your doctor to find out the cause of your symptoms. The right treatment plan can restore balance in your body and help you feel like yourself again.
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By Nicole Galan, RN
Nicole Galan, RN, is a registered nurse and the author of “The Everything Fertility Book.”