Glandular System: Definition, Main Types, Endocrine Organs, Function and Clinical Significance

More commonly known as the endocrine system, it is a chemical messenger system that consists of hormones.

The group of glands in an organism transport these hormones directly to the circulatory system to transport them to distant target organs, and the feedback loops of homeostasis that drive the hormones.

In humans, the main endocrine glands are the thyroid gland and the adrenal glands. In vertebrates, the hypothalamus is the neuronal control center for all endocrine systems .

The field of study that deals with the endocrine system and its disorders is endocrinology, a branch of internal medicine.

The special characteristics of the endocrine glands are, in general, their ductless nature, their vascularity, and commonly the presence of intracellular vacuoles or granules that store their hormones.

In contrast, exocrine glands, such as salivary glands, sweat glands, and glands of the gastrointestinal tract, tend to be much less vascular and have ducts or a hollow lumen.

Several glands pointing to each other are commonly referred to as the axis, for example, the hypothalamic-pituitary-adrenal axis.

In addition to the specialized endocrine organs mentioned above, many other organs that are part of other body systems, such as the bones, kidneys, liver, heart, and gonads, have secondary endocrine functions.

For example, the kidney secretes endocrine hormones such as erythropoietin and renin. Hormones can consist of complexes of amino acids, steroids, eicosanoids, leukotrienes, or prostaglandins.

The endocrine system is in contrast to the exocrine system, which secretes its hormones out of the body through ducts.

Unlike endocrine factors that travel considerably longer distances through the circulatory system, other signaling molecules, such as paracrine factors involved in paracrine signaling, diffuse over a relatively short distance.

The word endocrine derives through New Latin from the Greek words ἔνδον, endon, “within” and “exocrine” from κρίνω, krīnō, “to separate, to distinguish.”

Types of major endocrine systems

The human endocrine system consists of several systems that operate through feedback loops. Several important feedback systems are mediated through the hypothalamus and pituitary.

Hypothalamic-pituitary-thyroid axis (TRH-TSH-T3 / T4)

The hypothalamic-pituitary-thyroid axis (HPT axis for short or quadratic thyroid homeostasis or thyrotropic feedback control) is part of the neuroendocrine system responsible for the regulation of metabolism.

As its name suggests, it depends on the hypothalamus, the pituitary gland, and the thyroid gland.

The hypothalamus detects low circulating levels of thyroid hormone ( triiodothyronine (T3) and thyroxine (T4) and responds by releasing thyrotropin-releasing hormone (TRH).

Thyrotropin-releasing hormone stimulates the anterior part of the pituitary to produce thyroid-stimulating hormone (TSH).

Thyroid stimulating hormone, in turn, stimulates the thyroid to produce thyroid hormone until blood levels return to normal.

Thyroid hormone exerts negative feedback control over the hypothalamus and anterior pituitary, thereby controlling the release of both thyrotropin-releasing hormone from the hypothalamus and thyroid-stimulating hormone from the anterior pituitary gland.

The hypothalamic-pituitary-adrenal axis (HPA axis or HTPA axis), the hypothalamic-pituitary-gonadal axis (HPG axis) and the hypothalamic-pituitary-thyroid axis are three pathways in which the direct neuroendocrine function of the hypothalamus and pituitary .

Gonadotropin Releasing Hormone, Luteinizing Hormone / Follicle Stimulating Hormone (GnRH – LH / FSH – Sex Hormones)

The hypothalamic-pituitary-gonadal axis (HPG axis) refers to the hypothalamus, the pituitary gland, and the gonadal glands as if these individual endocrine glands were a single entity.

Because these glands often act in concert, physiologists and endocrinologists find it convenient and descriptive to speak of them as a single system.

The hypothalamic-pituitary-gonadal axis plays a critical role in the development and regulation of various body systems, such as the reproductive and immune systems.

Fluctuations on this axis cause changes in the hormones produced by each gland and have various local and systemic effects in the body.

The axis controls development, reproduction, and aging in animals. Gonadotropin-releasing hormone (GnRH) is secreted from the hypothalamus by neurons that express gonadotropin-releasing hormone.

The anterior portion of the pituitary gland produces luteinizing hormone (LH) and follicle-stimulating hormone (FSH), and the gonads produce estrogen and testosterone.

In oviparous organisms (eg, fish, reptiles, amphibians, birds), the hypothalamic-pituitary-gonadal axis is commonly referred to as the hypothalamic-pituitary-gonadal liver axis (HHHG axis) in women.

Many egg yolks and chorionic proteins are heterologously synthesized in the liver, which are necessary for the growth and development of oocytes. Examples of such necessary liver proteins are vitellogenin and choriogenin.

The hypothalamic-pituitary-adrenal, hypothalamic-pituitary-gonadal and hypothalamic-pituitary-thyroid axes are three pathways in which direct neuroendocrine function of the hypothalamus and pituitary.

Hypothalamic-pituitary-adrenal axis (CRH-ACTH-cortisol)

The hypothalamic-pituitary-adrenal axis (HPA axis or HTPA axis) is a complex set of direct influences and feedback interactions between three components:

The hypothalamus, the pituitary gland (a pea-shaped structure below the thalamus), and the adrenal (also called the “adrenal” glands) (small, conical organs in the upper part of the kidneys).

These organs and their interactions make up the hypothalamic-pituitary-adrenal axis, an important neuroendocrine system that controls reactions to stress and regulates many bodily processes, including digestion, the immune system, mood and emotions, sexuality, and energy storage and expenditure.

It is the common mechanism for interactions between the glands, hormones, and parts of the midbrain that mediate general adaptation syndrome (GAS).

While steroid hormones are produced primarily in vertebrates, the physiological role of the hypothalamic-pituitary-adrenal axis and corticosteroids in the stress response is so fundamental that analogous systems can also be found in invertebrates and single-cell organisms.

The hypothalamic-pituitary-adrenal axis, the hypothalamic-pituitary-gonadal axis, the hypothalamic-pituitary-thyroid axis, and the hypothalamic-neurohypophyseal system are the four major neuroendocrine systems through which the hypothalamus and pituitary direct neuroendocrine function.

Renin – angiotensin – aldosterone

The renin-angiotensin system (RAS) or the renin-angiotensin-aldosterone system (RAAS) is a hormonal system that regulates blood pressure and water balance.

When renal blood flow is reduced, juxtaglomerular cells in the kidneys convert the precursor prorenin, which is already present in the blood, into renin and secrete it directly into the circulation.

Plasma renin then carries out the conversion of angiotensinogen, released by the liver, to angiotensin I.

Angiotensin I is subsequently converted to angiotensin II by angiotensin converting enzyme (ACE) found in the lungs.

Angiotensin II is a powerful vasoconstrictive peptide that causes blood vessels to narrow, resulting in increased blood pressure.

Angiotensin II also stimulates the secretion of the hormone aldosterone from the adrenal cortex.

Aldosterone causes the kidney tubules to increase the reabsorption of sodium and water into the blood, while at the same time causing the excretion of potassium (to maintain electrolyte balance).

This increases the volume of extracellular fluid in the body, which also increases blood pressure.

If the renin-angiotensin system is abnormally active, the blood pressure will be too high. There are many medications that interrupt different steps in this system to lower blood pressure.

These drugs are one of the main ways to control high blood pressure, heart failure, kidney failure, and the harmful effects of diabetes.

Leptin vs. Insulin

In biology, energy homeostasis, or homeostatic control of energy balance, is a biological process that involves the coordinated homeostatic regulation of food intake (energy input) and energy expenditure (energy output).

The human brain, particularly the hypothalamus, plays a central role in the regulation of energy homeostasis and generates the sensation of hunger by integrating a series of biochemical signals that transmit information about energy balance.

Fifty percent of the energy from glucose metabolism is immediately converted into heat. Energy homeostasis is an important aspect of bioenergetics.


Endocrine glands are glands of the endocrine system that secrete their products, hormones, directly into interstitial spaces and are then absorbed into the blood rather than through a duct.

The major glands of the endocrine system include the pineal gland, pituitary gland, pancreas, ovaries, testes, thyroid gland, parathyroid gland, hypothalamus, and adrenal glands. The hypothalamus and the pituitary gland are neuroendocrine organs.

Main endocrine organs

Pituitary gland

The pituitary gland hangs from the base of the brain by a stalk and is surrounded by bone.

It consists of a hormone-producing glandular portion (anterior pituitary) and a neural portion (posterior pituitary), which is an extension of the hypothalamus.

The hypothalamus regulates the hormonal production of the anterior pituitary and creates two hormones that it exports to the posterior pituitary for storage and subsequent release.

Four of the six hormones of the anterior pituitary are tropical hormones that regulate the function of other endocrine organs.

Most anterior pituitary hormones exhibit a diurnal rhythm of release, which is subject to modification by stimuli influencing the hypothalamus.

Somatotropic hormone or growth hormone (GH) is an anabolic hormone that stimulates the growth of all tissues in the body, but especially skeletal muscles and bones. It can act directly or indirectly through insulin-like growth factors (IGFs).

Growth hormone mobilizes fats, stimulates protein synthesis, and inhibits glucose absorption and metabolism.

Secretion is regulated by growth hormone releasing hormone (GHRH) and growth hormone inhibiting hormone (GHIH) or somatostatin.

Hypersecretion causes gigantism in children and acromegaly in adults; hyposecretion in children causes pituitary dwarfism.

Thyroid stimulating hormone (HST) promotes the normal development and activity of the thyroid gland. Thyrotropin-releasing hormone (TRH) stimulates its release; negative feedback from thyroid hormone inhibits it.

Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to release corticosteroids.

The release of corticotropin-releasing hormone (HLC) triggers the release of corticotropin-releasing hormone and inhibits the increase in glucocorticoid levels.

Gonadotropin follicle stimulating hormone and luteinizing hormone regulate the functions of the gonads in both sexes.

Follicle-stimulating hormone stimulates the production of sex cells; Luteinizing hormone stimulates the production of gonadal hormones.

Gonadotropin levels increase in response to gonadotropin-releasing hormone. Negative feedback from gonadal hormones inhibits gonadotropin release.

Prolactin (PRL) promotes milk production in women. Its secretion is caused by prolactin-releasing hormone (HLP) and inhibited by prolactin-inhibiting hormone (HIP).

The neurohypophysis stores and releases two hypothalamic hormones:

Oxytocin stimulates strong uterine contractions, which trigger the delivery of a baby, and the ejection of milk in lactating women. Its release is reflexively mediated by the hypothalamus and represents a positive feedback mechanism.

Antidiuretic hormone (ADH) stimulates the renal tubules to reabsorb and conserve water, resulting in small volumes of highly concentrated urine and decreased plasma osmolarity.

Antidiuretic hormone is released in response to high concentrations of solute in the blood and inhibited by low concentrations of solute in the blood. Hyposecretion produces diabetes insipidus.

Thyroid gland

The thyroid gland is located in the front of the neck, opposite the thyroid cartilage, and is shaped like a butterfly, with two wings connected by a central isthmus.

Thyroid tissue consists of follicles with stored protein called a colloid, which contains thyroglobulin, a precursor to other thyroid hormones, which are made in the colloid.

Thyroid hormones increase the rate of cellular metabolism and include thyroxine (T4) and triiodothyronine (T3). The secretion is stimulated by thyroid stimulating hormone, secreted by the anterior pituitary.

When thyroid levels are high, there is negative feedback that decreases the amount of thyroid stimulating hormone secreted. Most of T4 is converted to T3 (a more active form) in target tissues.

Calcitonin, produced by the parafollicular cells of the thyroid gland in response to increased blood calcium levels, depresses blood calcium levels by inhibiting bone matrix resorption and increasing calcium deposition in bone.

Parathyroid glands

The parathyroid glands, of which there are 4-6, are located at the back of the thyroid glands and secrete parathyroid hormone (PTH), which causes an increase in calcium levels in the blood by attacking the bone, intestine and the kidneys.

Parathyroid hormone is the antagonist of calcitonin. The release of parathyroid hormone is triggered by the drop in calcium levels in the blood and is inhibited by the increase in calcium levels in the blood.

Kidney glands

The adrenal glands are located above the kidneys in humans and in front of the kidneys in other animals. The adrenal glands produce a variety of hormones including adrenaline and the steroids aldosterone and cortisol.

Control behavior during crisis and emotional situations. Stimulates the heart and its conductive tissues and metabolic processes.


The pancreas, located in the abdomen, below and behind the stomach, is an exocrine gland and an endocrine gland. Alpha and beta cells are the endocrine cells in the pancreatic islets that release insulin and glucagon and smaller amounts of other hormones into the blood.

Insulin and glucagon influence blood sugar levels. Glucagon is released when the blood glucose level is low and stimulates the liver to release glucose into the blood.

Insulin increases the rate of glucose absorption and metabolism by most cells in the body.

Somatostatin is released by Delta cells and acts as an inhibitor of growth hormone, insulin, and glucagon.


The female’s ovaries, located in the pelvic cavity, release two main hormones. Estrogen secretion by ovarian follicles begins at puberty under the influence of follicle-stimulating hormone.

Estrogens stimulate the maturation of the female reproductive system and the development of secondary sexual characteristics.

Progesterone is released in response to elevated levels of luteinizing hormone in the blood. It works with estrogens to establish the menstrual cycle.

A man’s testes begin to produce testosterone at puberty in response to luteinizing hormone.

Testosterone promotes the maturation of the male reproductive organs, the development of secondary sexual characteristics, and the production of sperm by the testes.

Pineal gland

The pineal gland is located in the diencephalon of the brain. It mainly releases melatonin, which influences daily rhythms and may have an antigonadotropic effect in humans. It can also influence melanotropes and melanocytes located in the skin.

Other hormone-producing structures

Many organs in the body that are not normally considered endocrine organs contain isolated clusters of cells that secrete hormones.

Examples include the heart (atrial natriuretic peptide); organs of the gastrointestinal tract (gastrin, secretin and others); the placenta (pregnancy hormones-estrogen, progesterone and others); the kidneys (erythropoietin and renin); the scam; skin (cholecalciferol); and adipose tissue (leptin and resistin).



A hormone is a class of signaling molecules produced by the glands in multicellular organisms that are transported through the circulatory system to target distant organs to regulate physiology and behavior.

Hormones have various chemical structures, mainly of 3 classes: eicosanoids, steroids, and amino acid / protein derivatives (amines, peptides, and proteins). The glands that secrete hormones comprise the endocrine system.

The term hormone is sometimes extended to include chemicals produced by cells that affect the same cell (autocrine or intracrine signaling) or nearby cells (paracrine signaling).

Hormones are used to communicate between organs and tissues for physiological regulation and behavioral activities, such as digestion, metabolism, respiration, tissue function, sensory perception, sleep, excretion, lactation, stress, growth and development, movement, reproduction, and mood. .

Hormones affect distant cells by binding to specific receptor proteins on the target cell resulting in a change in cell function.

When a hormone binds to the receptor, it results in the activation of a signal transduction pathway.

This can lead to cell-type specific responses including rapid non-genomic effects or slower genomic responses where hormones acting through their receptors activate gene transcription resulting in increased expression of target proteins.

Hormones based on amino acids (amines and peptide or protein hormones) are soluble in water and act on the surface of target cells through second messengers.

Steroid hormones, being fat soluble, move through the plasma membranes of target cells (both cytoplasmic and nuclear) to act within their nuclei.

Cell signaling

The typical mode of cell signaling in the endocrine system is endocrine signaling, that is, the use of the circulatory system to reach distant target organs. However, there are other modes as well, namely paracrine, autocrine, and neuroendocrine signaling.

Purely neurocrine signaling between neurons, on the other hand, belongs entirely to the nervous system.


Autocrine signaling is a way of indicating that a cell secretes a hormone or chemical messenger (called an autocrine agent) that binds to autocrine receptors on the same cell, causing changes in the cells.


Some endocrinologists and physicians include the paracrine system as part of the endocrine system, but there is no consensus. Paracrines are slower acting and target cells of the same tissue or organ.

An example of this is somatostatin which is released by some pancreatic cells and targets other pancreatic cells.


Juxtacrine signaling is a type of cell-to-cell communication that is transmitted through oligosaccharide, lipid, or protein components of a cell membrane, and can affect the sending cell or immediately adjacent cells.

It occurs between adjacent cells that possess broad patches of closely opposite plasma membrane linked by transmembrane channels known as connexons. The space between cells can generally be between only 2 and 4 nm.

Clinical significance


Diseases of the endocrine system are common, including conditions such as diabetes mellitus, thyroid disease, and obesity.

Endocrine disease is characterized by irregular hormone release (a productive pituitary adenoma), an inadequate response to signaling (hypothyroidism).

Gland failure (type 1 diabetes mellitus, decreased erythropoiesis in chronic renal failure) or structural enlargement at a critical site such as the thyroid (toxic multinodular goiter).

Hypofunction of the endocrine glands can occur as a result of loss of reserve, hyposecretion, agenesis, atrophy, or active destruction.

Hyperfunction can occur as a result of hypersecretion, loss of suppression, hyperplastic or neoplastic change, or hyperstimulation.

Endocrinopathies are classified as primary, secondary, or tertiary. Primary endocrine disease inhibits the action of the downstream glands.

Secondary endocrine disease is indicative of a problem with the pituitary gland. Tertiary endocrine disease is associated with dysfunction of the hypothalamus and its releasing hormones.

As the thyroid and hormones have been implicated in signaling the proliferation of distant tissues, for example, the estrogen receptor has been shown to be involved in certain breast cancers.

Endocrine, paracrine, and autocrine signaling have been implicated in proliferation, one of the necessary steps in oncogenesis.

Other common diseases that result from endocrine dysfunction include Addison’s disease, Cushing’s disease, and Graves’ disease.

Cushing’s disease and Addison’s disease are pathologies that involve dysfunction of the adrenal gland.

Dysfunction in the adrenal gland can be due to primary or secondary factors and can lead to hypercortisolism or hypocartisolism.

Cushing’s disease is characterized by hypersecretion of adrenocorticotropic hormone (ACTH) due to a pituitary adenoma that ultimately causes endogenous hypercortisolism by stimulating the adrenal glands.

Some clinical signs of Cushing’s disease include obesity, moon face, and hirsutism. Addison’s disease is an endocrine disease that results from hypocartisolism caused by the insufficiency of the adrenal glands.

Adrenal insufficiency is important because it correlates with decreased ability to maintain blood pressure and blood sugar, a defect that can be fatal.

Graves’ disease involves an overactive thyroid gland that produces the hormones T3 and T4.

The effects of Graves’ disease range from excessive sweating, fatigue, heat intolerance, and high blood pressure to eye swelling that causes redness, swelling, and, in rare cases, reduced or double vision.

Other animals

A neuroendocrine system has been observed in all animals with a nervous system, and all vertebrates have a hypothalamic-pituitary axis.

All vertebrates have a thyroid, which in amphibians is also crucial for the transformation of larvae into adults.

All vertebrates have adrenal gland tissue, and mammals are unique in organizing it into layers. All vertebrates have some type of renin-angiotensin axis, and all tetrapods have aldosterone as their primary mineralocorticoid.