The Endocrine System

The endocrine system uses hormones to coordinate body functions. It is a network of glands that releases chemical messengers into the body. These hormones carry signals between secretory cells and distant target cells. A hormone only affects cells that have the right receptor for it.

The endocrine system helps maintain homeostasis, or a stable internal balance. It works with the nervous system to regulate metabolism, reproduction, and development. Both systems support intercellular...

The endocrine system is made of glands, tissues, and organs spread across the body. These parts work together to release hormones, which are chemical signals that help control body functions. Endocrine glands are ductless glands, so they release hormones directly into the interstitial fluid, the fluid around cells.

The hypothalamus is a key neuroendocrine organ in the brain. It links the nervous system and endocrine system and helps regulate hormones. Near it are the pituitary gland and pineal...

Hormones control cell activity by binding to receptors and starting a signaling cascade. A signaling cascade is a chain of events that carries information within or between cells and ends with a specific response in the target cell. In the endocrine system, hormone action depends on whether the hormone is lipid-soluble or water-soluble.

Lipid-soluble hormones, such as steroid hormones, act inside the cell. Because they are lipid-based, they can pass through cell membranes. They then bind to...

Hormones can be grouped by their structure and how they act in the body. The main groups are steroids, eicosanoids, amino acid-based derivatives, and peptide hormones. These categories help explain why some hormones travel easily through cell membranes while others act in more local ways.

Steroids and eicosanoids are lipid-soluble hormones, which means they dissolve in fats. Steroids come from cholesterol and have four linked carbon rings with different side chains. Examples include estradiol...

Water-soluble hormones signal through cell receptors and second messengers. Because these hormones cannot cross the plasma membrane, they bind to protein receptors that span the membrane and start signaling inside the cell. The response often depends on small messenger molecules such as cAMP or calcium ions.

Many of these hormones use G protein-coupled receptors, or GPCRs. These membrane receptors connect to regulatory G proteins, which then activate enzymes such as adenylyl cyclase or...

Hormones trigger cell responses by binding to receptors on the cell surface or inside the target cell. This binding starts a chain of events that changes how the cell behaves. The response depends on the hormone, the receptor, and the type of target cell.

Cells can also change how strongly they respond by adjusting receptor numbers. When a cell is exposed to high hormone levels for a long time, it may down-regulate receptors. Down-regulation means the cell makes fewer receptors for that...

Hormone secretion stays in balance through neural, humoral, and hormonal signals. These signals tell endocrine glands when to release hormones. Together, they help the body respond to changing conditions.

Neural stimuli begin with environmental cues that trigger action potentials in nerve fibers. These nerve signals travel to their targets and can quickly start hormone release. A clear example is stress, when the sympathetic nervous system causes the adrenal glands to release epinephrine. This...

The pituitary gland is a small endocrine organ in the sphenoid bone, just under the hypothalamus. In adults, it has two main regions: the anterior lobe and the posterior lobe. During fetal development, a third region called the pars intermedia forms, but it atrophies and disappears. Some of its cells remain and are found next to the anterior pituitary in adults.

The anterior lobe includes the pars distalis and the pars tuberalis. These regions are made of glandular epithelial tissue, which...

The pituitary gland is a small, pea-sized gland at the base of the brain. It helps regulate body functions such as growth and reproduction. The gland has two lobes, the anterior lobe and the posterior lobe. In the anterior lobe, secretory cell clusters in the pars distalis are controlled by hypothalamic regulators and produce six main hormones.

Growth hormone is the most abundant hormone released from the anterior lobe. It supports overall growth by helping the body produce insulin-like growth...

The thyroid gland is a small, butterfly-shaped gland in the neck. It lies over the front surface of the trachea. The gland has two lateral lobes joined by a thin band of tissue called the isthmus.

Inside each lobe are many small, round thyroid follicles. These follicles are surrounded by a network of blood vessels. Each follicle has a central cavity lined by simple cuboidal to squamous epithelial cells called follicular cells.

Follicular cells make the glycoprotein thyroglobulin and store it...

Thyroid hormones T3 and T4 are made through a step-by-step process in the thyroid gland. Their release is also controlled by a feedback loop that helps keep hormone levels balanced in the body.

When blood levels of T3 and T4 drop, the hypothalamus responds by releasing thyrotropin-releasing hormone, or TRH. TRH travels to the pituitary gland and stimulates it to release thyroid-stimulating hormone, or TSH, into the bloodstream.

TSH then acts on the thyroid gland. It stimulates the follicular...

Thyroid hormones help control growth, metabolism, and body balance. They are essential during fetal life and childhood, when the skeletal, muscular, and nervous systems are developing and maturing. They also support bone mineral turnover and protein synthesis in growing tissues.

In adulthood, thyroid hormones continue to affect the body in important ways. They help regulate gonadal hormone secretion, especially in females, and support endocrine tissue function. They also contribute to normal...

Calcitonin is a polypeptide hormone that helps control calcium levels in body fluids. It is released by parafollicular cells, also called C cells, in the follicular epithelium of the thyroid gland. Calcitonin responds to changes in blood calcium levels. It is also influenced by gastrointestinal hormones such as gastrin and cholecystokinin.

The exact mechanism of calcitonin in calcium homeostasis is still not fully clear. Even so, its role is important in several body processes. In childhood,...

The parathyroid glands help control calcium balance in the body. These two pairs of glands sit on the posterior surface of the thyroid gland and are enclosed by a dense capsule. They contain two main cell types: principal cells and oxyphil cells.

Principal parathyroid cells make parathyroid hormone, or PTH, also called parathormone. These cells sense when blood calcium levels fall. When calcium drops, they release PTH to start a response that helps restore calcium balance.

PTH acts on bones,...

The adrenal glands help the body respond to stress. These paired, pyramid-shaped glands sit above the kidneys and line up with the twelfth rib. They also have a yellow tint because they store cholesterol and fatty acids needed to make hormones.

Each gland is wrapped in a fibrous capsule and cushioned by fat. Inside, the gland has two main regions with different jobs. The outer adrenal cortex makes corticosteroids, which support longer-lasting stress responses.

The cortex is organized into...

Adrenal hormones help control electrolyte balance and the body’s stress response. They come from the adrenal cortex and the adrenal medulla, which work together to support survival and normal body function.

The adrenal cortex makes over two dozen corticosteroid hormones. In the zona glomerulosa, the main mineralocorticoid is aldosterone. Mineralocorticoids help control the electrolyte makeup of body fluids.

The zona fasciculata produces glucocorticoids such as cortisol and corticosterone.

Hormone imbalances in the adrenal glands can cause too little or too much hormone production. The adrenal glands make hormones that help control stress, blood pressure, and body balance. When these levels move away from normal, several adrenal disorders can develop.

Adrenal insufficiency happens when the adrenal cortex makes too little cortisol and aldosterone. A common example is Addison’s disease. It can cause skin bronzing, dehydration, low blood pressure, fatigue, and weight loss.

The pineal gland is a small endocrine gland in the forebrain. It sits on top of the third ventricle in the diencephalon. It gets its pinecone-shaped name from its form.

This gland is made of secretory cells called pinealocytes. These cells are arranged in tight cords and clusters around dense particles of calcium salts. The pineal gland mainly releases melatonin, a hormone made from serotonin.

Melatonin levels in the blood follow a day-night cycle. Light exposure affects this pattern.

The pancreas uses two main kinds of cells to support digestion and blood sugar control. Its exocrine cells and endocrine cells work together to keep the body balanced. The pancreas is a glandular organ in the abdominal cavity, and it belongs to both the digestive and endocrine systems.

Exocrine function comes from acinar cells, which are grouped into clusters called acini. These cells release large amounts of alkaline digestive juice that is rich in enzymes. This secretion helps support...

Insulin and glucagon are pancreatic hormones that help keep blood glucose levels in balance. Insulin is released by beta cells when blood glucose is high. Glucagon is released by alpha cells when blood glucose drops below normal.

Insulin helps glucose move into insulin-dependent cells that have insulin receptors on their plasma membranes. It increases the number of glucose transport proteins in the cell membrane, which allows glucose to enter the cell more easily. This raises glucose use and...

Diabetes mellitus is a chronic metabolic disorder that raises blood glucose levels. It happens when the body does not make enough insulin, when cells do not respond well to insulin, or when both problems are present. This condition can affect health and quality of life in many people worldwide.

Type 1 diabetes is an autoimmune disease. In this form, the immune system attacks and destroys the beta cells in the pancreas that produce insulin. Because of this damage, the body cannot make enough...

Type 2 diabetes and gestational diabetes both involve problems with blood sugar control. In Type 2 diabetes, insulin resistance develops when insulin receptors on cells lose their response to insulin. This lowers the cell’s ability to take up glucose and leaves blood glucose levels high.

Doctors use a series of blood glucose tests to diagnose Type 2 diabetes. These tests show whether blood glucose is in the normal range. If results are abnormal, a patient may be labeled prediabetic or...

Gonadal and placental hormones guide sexual development, reproduction, and pregnancy. The gonads are the testes in males and the ovaries in females. They produce hormones that help control these body processes.

In males, testosterone is the main gonadal androgen, or male sex hormone. It helps the penis and testes mature. It also drives secondary sex characteristics, such as a deeper voice and facial and pubic hair growth. Testosterone also supports sperm production.

In females, estrogen and...