What is the endocrine system?

The endocrine system is one of two major regulatory systems that release chemicals in the body (the other one is the nervous system; for more, see the chapter “How Diabetes Affects the Nervous System”). The endocrine system, together with the nervous system, controls and coordinates the functions of all of the human body systems. It contains a group of glands that secrete hormones, all of which help maintain metabolic functions, allow the body to react to stress, and regulate growth, reproduction, and nutrient use by the body’s cells.

What are hormones and hormone receptors?

Hormones, or chemicals made and secreted by endocrine glands, are the main messengers of the endocrine system. Hormones are transported in the bloodstream to all parts of the body and interact with target cells (cells that contain hormone receptors and respond to a specific hormone in the body), which regulate metabolic rate (including glucose), growth, maturation, and reproduction. In most cases, hormones produce a specific effect on the activity of cells that are remotely located from the hormones’ point of origin. Hormone receptors are located either on the surface of a cell’s outer membrane or inside the cell itself (hormone receptors and hormones fit together much like a lock and key).

What are the major endocrine glands and their respective hormones?

The major endocrine glands are the pituitary, thyroid, parathyroid, pineal, and adrenal glands. Other hormone-secreting organs are the central nervous system (hypothalamus), kidneys, heart, pancreas, thymus, ovaries, and testes. Some organs, such as the pancreas, secrete hormones as an endocrine function but also have other functions. The following lists some of the major glands, their target tissues, and their principal function in the body (those directly associated with diabetes are highlighted):

Endocrine Glands and Their Functions

Endocrine Glands and Their Functions

Endocrine Gland/Hormone

Target Tissue

Principal Function

Posterior pituitary

Antidiuretic hormone (ADH)


Stimulates water reabsorption by kidneys


Uterus, mammary

Stimulates uterine contractions and glands milk ejection

Anterior pituitary

Growth hormone (GH)


Stimulates growth, especially cell division and bone growth

Adrenocorticotropichormone (ACTH)

Adrenal cortex

Stimulates adrenal cortex

Thyroid-stimulating (TSH)

Thyroid gland

Stimulates thyroid hormone

Luteinizing (LH)


Stimulates ovaries and testes hormone

Follicle-stimulating (FSH)


Controls egg and sperm production

Prolactin (PRL)


Stimulates milk production

Melanocyte-stimulating (MSH)


Regulates skin color in reptiles and hormone amphibians, but has an unknown function in humans




Lowers blood-calcium level


Parathyroid hormone (PTH)

Bone, kidneys, digestive tract

Raises blood-calcium level

Adrenal medulla

Epinephrine (adrenaline) and norepinephrine (noradrenaline)

Skeletal muscle, cardiac muscle blood vessels

Initiates stress responses; raises heart rate, blood pressure, metabolic rates; constricts certain blood vessels

Adrenal cortex


Kidney tubules

Stimulates kidneys to reabsorb sodium and excrete potassium



Increases blood glucose




Lowers blood glucose level; stimulates formation and storage of glycogen


Liver, adipose tissue

Raises blood glucose level



General; female reproductive structures

Stimulates development of secondary sex characteristics in females and uterine lining


Uterus, breasts

Promotes growth of uterine lining; stimulates breast development


Androgens (testosterone)

General; male reproductive organs

Stimulates development of male sex structures and spermatogenesis

Pineal gland


Gonads, pigment cells

Involved in daily and seasonal rhythmic activities (circadian cycles); influences pigmentation in some species

The endocrine glands excrete various hormones into the body that control various functions, such as water absorption, growth, and calcium levels in the blood.

What is the hormonal response to stress and its connection to blood glucose?

The stress response has three basic phases: the alarm phase, the resistance phase, and the exhaustion phase. The alarm phase is an immediate reaction to stress, with epinephrine the dominant hormone. It is released along with activation of the sympathetic nervous system and produces the “fight or flight” response. Nonessential body functions such as digestive, urinary, and reproductive activities are inhibited. The resistance phase follows the alarm phase if the stress lasts more than several hours. Glucocorticoids (see below) are the dominant hormones of the resistance phase. Endocrine secretions maintain levels of glucose in the blood by moving fat and protein reserves, conserving glucose for nerve tissues, and synthesizing and releasing glucose by the liver. If the body does not overcome the stress during the resistance phase, the exhaustion phase begins. Prolonged exposure to high levels of hormones involved in the resistance phase can lead to the collapse of vital organ systems.

How does the hormone adiponectin affect blood glucose?

Adiponectin is a hormone that is involved in regulating blood glucose levels. Medically, it is called a protein hormone that is produced and secreted exclusively by the fat cells (called adipocytes). These fat cells are responsible for regulating the metabolism of lipids (fats) and glucose. Thus, this hormone influences the body’s response to insulin and also has an anti-inflammatory effect on the cells lining blood vessel walls. It is usually found in high levels in the bodies of people who are not obese. It can also be found in people who are obese and some who are overweight (but at much lower levels) and often in people with insulin resistance and type 2 diabetes.


What are the physical characteristics of the adrenal glands?

The adrenal (from the Latin, meaning “upon the kidneys”) glands sit on the upper tip of each kidney. Each adrenal gland weighs approximately 0.19 ounces (7.5 grams). The glands are yellow in color and have a pyramid shape. Each adrenal gland has two sections that may almost be considered as separate glands. The inner portion is the adrenal medulla (from the Latin, meaning “marrow”). The outer portion, which surrounds the adrenal medulla, is the adrenal cortex (from the Latin, meaning “bark,” because its appearance is similar to the outer covering of a tree). The adrenal cortex is the larger part of the adrenal glands, accounting for nearly 90 percent of the gland by weight.

What are the functions of corticosteroids?

The adrenal cortex secretes more than two dozen different steroid hormones called the adrenocortical steroids, or simply corticosteroids. The corticosteroids are vital for life and well-being, with each serving a unique purpose. The following lists the corticosteroids and their functions, including those associated with blood glucose and glycogen (which, in turn, are associated with diabetes):

Corticosteroids and Their Functions






Increases reabsorption of sodium ions and water from the urine; stimulates loss of potassium ions through excretion of urine


Most cells

Releases amino acids from skeletal muscles, lipids from adipose (fat) tissues; promotes liver glycogen and glucose formation; promotes peripheral utilization of lipids; anti-inflammatory effects


Promotes growth of pubic hair in boys and girls; in adult women, promotes muscle mass, blood cell formation, and supports the libido; in adult men, adrenal androgens are less significant because androgens are released primarily from the gonads

What are the major glucocorticoid hormones and their diabetes connections?

Cortisol, corticosterone, and cortisone are the three most important glucocorticoid hormones in the body. Cortisol, also called hydrocortisone, is the most abundant glucocorticoid produced, accounting for nearly 95 percent of the activity of the glucocorticoids. These hormones have many varying effects on the body. In terms of diabetes, effects include the stimulation of glucose synthesis and glycogen formation, especially within the liver. They also stimulate the release of fatty acids from adipose tissue, which can be used as an energy source; decrease the effects of physical and emotional stress (such as fright, bleeding, and infection, since the added glucose from the liver gives tissues a ready source of energy); suppress allergic and inflammatory reactions (which is important to a person with diabetes, as inflammation can lead to other problems); and decrease and suppress the activities of white blood cells and other components of the immune system.

The adrenal glands rest on top of the kidneys; they produce hormones that affect such things as blood pressure and the body’s response to stress.


What is the thyroid?

The thyroid is a butterfly-shaped gland in the neck. It is located just below the projection at the front of the neck (called the Adam’s apple in men) and above the collarbone. Its major task is to regulate metabolism of the heart, liver, muscles, and other organs through the release of two hormones, thyroxine (T4) and triiodothyronine (T3). The gland also is part of a feedback mechanism that includes the pituitary gland (in the brain) and the hypothalamus (an area of the brain).

What are T3, T4, and TSH and their functions in the body?

The thyroid itself has hormones and stimulates others to produce substances within certain areas of the body. These thyroid hormones have wide-ranging effects in regulating the function of virtually every organ, including how the body uses energy. Consequently, any changes in the thyroid hormone levels in the blood can affect many areas of the body and cause a wide range of symptoms if they are not balanced. In general, there is a sequence to the release of thyroid hormones. First, the thyroid-stimulating hormone (TSH) is produced when the brain’s hypothalamus releases a substance called thyrotropin-releasing hormone (TRH). The TRH then triggers the pituitary gland to release the TSH. This release causes the thyroid to produce two hormones, triiodothyronine (T3) and thyroxine (T4), both of which help control the body’s metabolism. From there, the brain’s pituitary gland keeps track of the thyroid hormone levels in the blood, increasing or decreasing the amount of TSH released to keep the levels balanced.

What are some statistics about people who have a thyroid disorder?

According to several thyroid organizations, millions of people have a thyroid disorder. Thyroid Federation International estimates that up to 300 million people worldwide have thyroid dysfunction, yet over half are unaware of their condition. According to the American Thyroid Association, it is thought that around 20 million Americans, or 12 percent of the population, have a thyroid disorder (although some research indicates the number may be as high as 27 million). Gender-wise, it is estimated that women are five to eight times more likely to have a thyroid disorder than men. It is often said that thyroid disorders are second only to diabetes as the most common condition that affects the human endocrine system.

What is the most common cause of hypothyroidism?

The most common cause of hypothyroidism is dietary iodine deficiency, with an estimated 200 million people having the condition (although this number differs by a few tens of thousands depending on the report). In the United States, there is a lower incidence of hypothyroidism because iodine is usually added to salt and foods that contain salt. The most common reason for hypothyroidism in the United States is Hashimoto thyroiditis-an inherited autoimmune condition in which the immune system mistakenly attacks the thyroid and which affects more than 14 million Americans.

Hyperthyroidism can manifest as a goiter (the enlargement of the thyroid), which inhibits the gland’s ability to process iodine.

What are hyperthyroidism and hypothyroidism?

If a person has an underactive thyroid gland, it is called hypothyroidism. (“Underactive” means producing lower amounts of thyroxine than normal; a test would show too much TSH, or thyroid-stimulating hormone). If a person’s thyroid gland is overactive (producing higher amounts of thyroxine than normal; a test would show too little TSH), it is called hyperthyroidism. This condition is less common than hypothyroidism. Neither condition is age related, but the conditions are often dependent on gender. For example, in the case of hyperthyroidism, it is often nine times more common in women than in men.

Is there a connection between diabetes and thyroid disorders?

Yes, according to several studies, including those from the American Diabetes Association, many people who have diabetes are often affected by a thyroid disorder. In fact, people with diabetes have an increased risk for thyroid problems. In the general population, about 6 to 8 percent have some type of thyroid disorder, but this figure increases to 10 percent in people who have diabetes. There also seems to be an autoimmune connection. When a person has one autoimmune disorder, such as type 1 diabetes, he or she has an increased risk for developing another such disease. For example, statistics indicate that around 3 percent of women with type 1 diabetes have some form of autoimmune thyroid disease. Another example is postpartum thyroiditis, a form of autoimmune thyroid disease that causes thyroid dysfunction within a few months after delivery of a child. It is three times more common in women with diabetes. As for type 2 diabetes, although it is not an autoimmune disease, there is an unexplained higher occurrence of thyroid problems, especially hypothyroidism. Some studies suggest that this may be because type 2 diabetes and thyroid disorders often develop as a person ages.

How are people with diabetes affected if they are hypothyroid or hyperthyroid?

Most studies indicate that, in general, if a person with diabetes has hypothyroidism, then there are few problems with blood glucose control, although the condition can reduce the removal of insulin from the bloodstream in people with type 1 diabetes (requiring that the dose of insulin be lower). If a person with diabetes has hyperthyroidism, he or she may have a more difficult time controlling blood glucose levels and may have to increase the insulin amount, as the condition causes an increase in glucose production in the liver, rapid absorption of glucose through the intestines, and increased insulin resistance. In addition, because diabetes can increase a person’s risk for heart disease, and hyperthyroidism often increases the heart rate, the condition may exacerbate certain heart conditions, such as angina, or interfere with the treatment of a heart condition.

What are the parathyroid glands?

There are four rice-grain-sized parathyroid glands located in the neck right behind the thyroid gland. Their main task is to keep calcium levels in the blood within a certain range by the release of the parathyroid hormone (PTH). The hormone not only helps muscles and nerves to work properly, but it is also connected to bone strength. In addition, if a person’s blood calcium is too high, the parathyroid glands make less PTH, lowering the amount of calcium by filtering it out of the blood in the kidneys. If the person’s blood calcium is low, the parathyroid glands make PTH, telling the body to increase the amount of the element in the blood. To do this, the body either absorbs more calcium from food in the intestines or it takes calcium from the bones.

Do people with diabetes have more problems with hyperparathyroidism than other people?

Yes, according to several studies, hyperparathyroidism-or a high level of the parathyroid hormone in the blood (it can contribute to a loss of calcium and thus weaken bones)-is more prevalent in people with diabetes than in the general population. In addition, the prevalence of diabetes in patients with hyperparathyroidism is higher than in the general population. Studies have also shown that hyperparathyroidism-whether the person has or doesn’t have diabetes-is almost three times more common in females than males and nearly seven times more common in postmenopausal women than in younger women.


Where is the pancreas located?

The pancreas (from the Greek, meaning “all flesh”) is located in what is called the abdominopelvic cavity, the area between and behind the stomach and the small intestine, and below the liver. It is an elongated organ that measures about 6 inches (12 to 15 centimeters) long.

Why is the pancreas often called a “mixed gland”?

The pancreas is often called a “mixed gland” because it has both endocrine and exocrine functions. As an endocrine gland, it secretes hormones into the bloodstream, including insulin and glucagon for blood glucose regulation. But only 1 percent of the weight of the pancreas serves as an endocrine gland. The remaining 99 percent has exocrine functions, especially in its relation to the body’s digestive system. (For more about the pancreas, including its functions as an exocrine gland, see the chapter “How Diabetes Affects the Digestive System.”)

The pancreas is located behind the stomach.

What are the pancreatic cells called the islets of Langerhans?

A cluster of cells in the pancreas is called the pancreatic islets (islets of Langerhans). They are also the cells that secrete hormones associated with blood glucose levels. In most adults, there can be between 200,000 and 2 million pancreatic islets scattered throughout the pancreas.

How many different types of cells are found in the islets of Langerhans?

The types of cells in each of the islets of Langerhans include the alpha, beta, delta, and F cells. The two most important types of cells-especially in terms of diabetes-are the alpha cells that produce glucagon for the body and the beta cells that produce insulin (for more about using islets in the future, especially to help curb type 2 diabetes, see the chapter “The Future and Diabetes”).

Why are alpha and beta cells in the pancreas so important to a person with-or without-diabetes?

The pancreas contains two types of cells that help with insulin stability: alpha and beta cells. Within the islets of Langerhans, the beta cells secrete insulin in response to increased blood glucose levels. This secretion stimulates the cells to take up glucose, providing energy to the body. The alpha cells secrete the hormone glucagon in response to a decrease in blood glucose levels, with the hormone binding to glucagon receptors in the liver. This secretion helps stimulate the breakdown of glycogen and release glucose into the bloodstream. (There are also other cell types in islets, including those that secrete the hormones somatostatin, ghrelin, and pancreatic polypeptide Y, but the alpha and beta cells are the most important to controlling the blood glucose levels in the body.)

The islets of Langerhans contain three types of cells that secrete insulin: alpha, beta, and delta cells.

Does any other chemical in the body stimulate the release of glucagon?

Yes. Although scientists know that glucose is the main factor in the release of glucagon, there are other factors including catecholamines (such as adrenaline) and amino acids (the “building blocks” of proteins) that also help stimulate glucagon release. In addition, insulin and fatty acids can inhibit the release of glucagon. There are many more factors, and scientists are currently trying to unravel the biology and hormone interactions in order to understand how cells respond (or don’t respond) when a person has diabetes.

How are the pancreas and insulin connected?

Insulin is made in the pancreas. As the blood glucose levels rise after a person eats a meal, the pancreas is triggered to release insulin. This hormone also enables the body’s cells to take in sugar and other nutrients. It is necessary for energy and growth, which is why an imbalance of the hormone in the body (which most often means diabetes) can have a dramatic effect on a person’s entire body system. When this system breaks down-in combination with other factors in the body-a person can develop insulin resistance, which is most often associated with type 2 diabetes. (For more about the pancreas, insulin resistance, and type 2 diabetes, see the chapter “Prediabetes and Type 2 Diabetes.”)

Who first described the pancreatic islets-and who discovered that the pancreas secreted insulin?

In the late 1860s, German pathologist and biologist Paul Langerhans (1847–1888) was the first to provide a detailed description of microscopic structures in the pancreas. What he found were uniquely shaped cells in the pancreas that were actually the islets. It was not until 1893 that French pathologist Gustave E. Laguesse (1861–1927) discovered that these polygon-shaped cells were actually the endocrine cells of the pancreas that secreted insulin.

What is the function of insulin in the body?

Insulin is secreted when blood glucose levels rise above normal. One of the most important tasks of insulin is to help with the transport of glucose across a cell’s membrane. In other words, insulin allows the glucose in the blood to diffuse into most body cells. It also stimulates the production of glycogen from glucose. The glucose is then stored in the liver to be released when blood glucose levels drop-for example, when the body needs energy during exercise. (For more about insulin, see the chapter “Taking Charge of Diabetes.”)

Does insulin just control the body’s glucose levels?

No, insulin has other duties besides controlling the body’s glucose levels. It also controls fat and helps the body store excess sugar as fat. Certain sugars and other carbohydrates with a high glycemic index are most likely to be stored as fat in the body. As the body’s sugar goes up and down before, during, and after meals, the insulin follows along, keeping the glucose levels even and contributing to the body’s fat stores. (For more about the glycemic index and how insulin and sugar are related to fat, see the chapter “Diabetes and Eating.”)