What Is the Endocrine System?

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• What Is the Endocrine System?
• Hypothalamus
• Pituitary
• Thyroid
• Parathyroids
• Adrenal Glands
• Pineal
• Reproductive Glands
• What Does the Endocrine System Do?
• Things That Can Go Wrong With the Endocrine System

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Ever dozed through chemistry class and wondered what chemistry had to do with you? A lot! Your body produces its own chemicals and uses them to control certain functions, and the main system that coordinates these chemicals is called the endocrine system.

What Is the Endocrine System?

Although we rarely think about the endocrine system, it influences almost every cell, organ, and function of our bodies. The endocrine system is instrumental in regulating mood, growth and development, tissue function, metabolism, and sexual function and reproductive processes.

In general, the endocrine system is in charge of body processes that happen slowly, such as cell growth. Faster processes like breathing and body movement are controlled by the nervous system. But even though the nervous system and endocrine system are separate systems, they often work together to help the body function properly.

The foundations of the endocrine system are the hormones and glands. As the body's chemical messengers, hormones (pronounced: hor-moanz) transfer information and instructions from one set of cells to another. Many different hormones move through the bloodstream, but each type of hormone is designed to affect only certain cells.

A gland is a group of cells that produces and secretes, or gives off, chemicals. A gland selects and removes materials from the blood, processes them, and secretes the finished chemical product for use somewhere in the body.

Some types of glands release their secretions in specific areas. For instance, exocrine (pronounced: ek-suh-krin) glands, such as the sweat and salivary glands, release secretions in the skin or inside the mouth. Endocrine glands, on the other hand, release more than 20 major hormones directly into the bloodstream where they can be transported to cells in other parts of the body.

The major glands that make up the human endocrine system include the:

• hypothalamus
• pituitary gland
• thyroid
• parathyroids
• adrenal glands
• pineal body
• reproductive glands (which include the ovaries and testes)

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Hypothalamus

The hypothalamus (pronounced: hi-po-tha-luh-mus), a collection of specialized cells that is located in the lower central part of the brain, is the main link between the endocrine and nervous systems. Nerve cells in the hypothalamus control the pituitary gland by producing chemicals that either stimulate or suppress hormone secretions from the pituitary.

Pituitary

Although it is no bigger than a pea, the pituitary (pronounced: puh-too-uh-ter-ee) gland, located at the base of the brain just beneath the hypothalamus, is considered the most important part of the endocrine system. It's often called the "master gland" because it makes hormones that control several other endocrine glands.

The production and secretion of pituitary hormones can be influenced by factors such as emotions and changes in the seasons. To accomplish this, the hypothalamus provides information sensed by the brain (such as environmental temperature, light exposure patterns, and feelings) to the pituitary.

The tiny pituitary is divided into two parts: the anterior lobe and the posterior lobe. The anterior lobe regulates the activity of the thyroid, adrenals, and reproductive glands. The anterior lobe produces hormones such as:

• growth hormone, which stimulates the growth of bone and other body tissues and plays a role in the body's handling of nutrients and minerals
• prolactin (pronounced: pro-lak-tin), which activates milk production in women who are breastfeeding
• thyrotropin (pronounced: thy-ruh-tro-pin), which stimulates the thyroid gland to produce thyroid hormones
• corticotropin (pronounced: kor-tih-ko-tro-pin), which stimulates the adrenal gland to produce certain hormones

The pituitary also secretes endorphins (pronounced: en-dor-fins), chemicals that act on the nervous system and reduce feelings of pain. In addition, the pituitary secretes hormones that signal the reproductive organs to make sex hormones. The pituitary gland also controls ovulation and the menstrual cycle in women.

The posterior lobe of the pituitary releases antidiuretic (pronounced: an-ty-dy-uh-reh-tik) hormone, which helps control the balance of water in the body. The posterior lobe also produces oxytocin (pronounced: ahk-see-toe-sin), which triggers the contractions of the uterus in a woman having a baby.

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Thyroid

The thyroid (pronounced: thy-royd), located in the front part of the lower neck, is shaped like a bow tie or butterfly and produces the thyroid hormones thyroxine (pronounced: thy-rahk-sin) and triiodothyronine (pronounced: try-eye-oh-doe-thy-ruh-neen). These hormones control the rate at which cells burn fuels from food to produce energy.

The production and release of thyroid hormones is controlled by thyrotropin (pronounced: thy-ruh-tro-pin), which is secreted by the pituitary gland. The more thyroid hormone there is in a person's bloodstream, the faster chemical reactions occur in the body.

Why are thyroid hormones so important? There are several reasons — for example, they help kids' and teens' bones grow and develop, and they also play a role in the development of the brain and nervous system in kids.

Parathyroids

Attached to the thyroid are four tiny glands that function together called the parathyroids (pronounced: par-uh-thy-roydz). They release parathyroid hormone, which regulates the level of calcium in the blood with the help of calcitonin (pronounced: kal-suh-toe-nin), which is produced in the thyroid.

Adrenal Glands

The body also has two triangular adrenal (pronounced: uh-dree-nul) glands, one on top of each kidney.

The adrenal glands have two parts, each of which produces a set of hormones and has a different function:

1. The outer part, the adrenal cortex, produces hormones called corticosteroids (pronounced: kor-tih-ko-ster-oydz) that influence or regulate salt and water balance in the body, the body's response to stress, metabolism, the immune system, and sexual development and function.
2. The inner part, the adrenal medulla (pronounced: muh-duh-luh), produces catecholamines (pronounced: kah-tuh-ko-luh-meenz), such as epinephrine (pronounced: eh-puh-neh-frun). Also called adrenaline, epinephrine increases blood pressure and heart rate when the body experiences stress.

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Pineal

The pineal (pronounced: pih-nee-ul) body, also called the pineal gland, is located in the middle of the brain. It secretes melatonin (pronounced: meh-luh-toe-nin), a hormone that may help regulate when you sleep at night and when you wake in the morning.

Reproductive Glands

The gonads are the main source of sex hormones. Most people don't realize it, but both guys and girls have gonads.

In guys the male gonads, or testes (pronounced: tes-teez), are located in the scrotum. They secrete hormones called androgens (pronounced: an-druh-junz), the most important of which is testosterone (pronounced: teh-stass-tuh-rone). These hormones tell a guy's body when it's time to make the changes associated with puberty, like penis and height growth, deepening voice, and growth in facial and pubic hair. Working with hormones from the pituitary gland, testosterone also tells a guy's body when it's time to produce sperm in the testes.

A girl's gonads, the ovaries (pronounced: oh-vuh-reez), are located in her pelvis. They produce eggs and secrete the female hormones estrogen (pronounced: es-truh-jen) and progesterone (pronounced: pro-jes-tuh-rone). Estrogen is involved when a girl begins to go through puberty. During puberty, a girl will experience breast growth, will begin to accumulate body fat around the hips and thighs, and will have a growth spurt. Estrogen and progesterone are also involved in the regulation of a girl's menstrual cycle. These hormones also play a role in pregnancy.

Although the endocrine glands are the body's main hormone producers, some other organs not in the endocrine system — such as the brain, heart, lungs, kidneys, liver, and skin — also produce and release hormones.

The pancreas (pronounced: pan-kree-us) is also part of the body's hormone-secreting system, even though it is also associated with the digestive system because it produces and secretes digestive enzymes. The pancreas produces (in addition to others) two important hormones, insulin (pronounced: in-suh-lin) and glucagon (pronounced: gloo-kuh-gawn). They work together to maintain a steady level of glucose, or sugar, in the blood and to keep the body supplied with fuel to produce and maintain stores of energy.

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What Does the Endocrine System Do?

Once a hormone is secreted, it travels from the endocrine gland that produced it through the bloodstream to the cells designed to receive its message. These cells are called target cells. Along the way to the target cells, special proteins bind to some of the hormones. These proteins act as carriers that control the amount of hormone that is available for the cells to use.

The target cells have receptors that latch onto only specific hormones, and each hormone has its own receptor, so that each hormone will communicate only with specific target cells that have receptors for that hormone. When the hormone reaches its target cell, it locks onto the cell's specific receptors and these hormone-receptor combinations transmit chemical instructions to the inner workings of the cell.

When hormone levels reach a certain normal amount, the endocrine system helps the body to keep that level of hormone in the blood. For example, if the thyroid gland has secreted the right amount of thyroid hormones into the blood, the pituitary gland senses the normal levels of thyroid hormone in the bloodstream. Then the pituitary gland adjusts its release of thyrotropin, the hormone that stimulates the thyroid gland to produce thyroid hormones.

Another example of this process is parathyroid hormone. Parathyroid hormone increases the level of calcium in the blood. When the blood calcium level rises, the parathyroid glands sense the change and reduce their secretion of parathyroid hormone. This turnoff process is called a negative feedback system.

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Things That Can Go Wrong With the Endocrine System

Too much or too little of any hormone can be harmful to your body. For example, if the pituitary gland produces too much growth hormone, a teen may grow excessively tall. If it produces too little, a teen may be unusually short. Doctors can often treat problems with the endocrine system by controlling the production of hormones or replacing certain hormones with medication.

Some endocrine problems that affect teens include:

Adrenal insufficiency. This condition occurs when the adrenal glands don't produce enough corticosteroids. The symptoms of adrenal insufficiency may include weakness, fatigue, abdominal pain, nausea, dehydration, and skin changes. Doctors treat adrenal insufficiency with medications to replace corticosteroid hormones.

Type 1 diabetes. When the pancreas fails to produce enough insulin, type 1 diabetes (pronounced: dy-uh-be-teez and previously known as juvenile diabetes) occurs. In kids and teens, type 1 diabetes is usually an autoimmune disorder, which means that some parts of the body's immune system attack and destroy the cells of the pancreas that produce insulin. To control their blood sugar levels and reduce the risk of developing diabetes problems, kids and teens with this condition need regular injections of insulin.

Type 2 diabetes. Unlike type 1 diabetes, in which the body can't produce normal amounts of insulin, in type 2 diabetes the body can't respond to insulin normally. Kids and teens with the condition tend to be overweight. Some kids and teens can control their blood sugar level with dietary changes, exercise, and oral medications, but many will need to take insulin injections like people with type 1 diabetes.

Growth hormone problems. Too much growth hormone in kids and teens who are still growing will make their bones and other body parts grow excessively. This rare condition (sometimes called gigantism) is usually caused by a pituitary tumor and can be treated by removing the tumor. The opposite can happen when a kid or teen has a pituitary glad that doesn't produce enough growth hormone. Doctors may treat these growth problems with medication.

Hyperthyroidism. Hyperthyroidism (pronounced: hy-per-thy-roy-dih-zum) is a condition in which the levels of thyroid hormones in the blood are very high. In kids and teens, the condition is usually caused by Graves' disease, an immune system problem that causes the thyroid gland to become very active. Doctors may treat hyperthyroidism with medications, surgery, or radiation treatments.

Hypothyroidism. Hypothyroidism (pronounced: hy-po-thy-roy-dih-zum) is a condition in which the levels of thyroid hormones in the blood are very low. Thyroid hormone deficiency slows body processes and may lead to fatigue, a slow heart rate, dry skin, weight gain, and constipation. Kids and teens with this condition may also grow more slowly and reach puberty at a later age. Hashimoto's thyroiditis is an immune system problem that often causes problems with the thyroid and blocks the production of thyroid hormone. Doctors often treat this problem with medication.

Precocious puberty. If the pituitary glands release hormones that stimulate the gonads to produce sex hormones too early, some kids may begin to go through puberty at a very young age. This condition is called precocious puberty. Kids and teens who are affected by precocious puberty can be treated with medication that will help them develop at a normal rate.

Reviewed by: Steven Dowshen, MD
Date reviewed: August 2009

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