Endocrine glands
Encyclopedia
Endocrine glands are glands of the endocrine system
that secrete their products, hormone
s, directly into the blood
rather than through a duct. The main endocrine glands include the pituitary gland
, pancreas
, ovaries
, testes
, thyroid gland
, and adrenal gland
s. The hypothalamus
is a neuroendocrine organ. Other organ
s which are not so well known for their endocrine activity include the stomach
, which produces such hormones as ghrelin
.
Local chemical messengers, not generally considered part of the endocrine system, include autocrine
s, which act on the cells that secrete them, and paracrine
s, which act on a different cell type nearby.
The ability of a target cell
to respond to a hormone depends on the presence of receptors, within the cell or on its plasma membrane, to which the hormone can bind.
Hormone receptors are dynamic structures. Changes in number and sensitivity of hormone receptors may occur in response to high or low levels of stimulating hormones.
Blood levels of hormones reflect a balance between secretion
and degradation/excretion
. The liver
and kidney
s are the major organs that degrade hormones; breakdown products are excreted in urine and feces.
Hormone half-life and duration of activity are limited and vary from hormone to hormone.
is the situation in which a hormone cannot exert its full effects without the presence of another hormone.
Synergism
occurs when two or more hormones produce the same effects in a target cell and their results are amplified.
Antagonism
occurs when a hormone opposes or reverses the effect of another hormone.
levels in the blood.
The nervous system
, acting through hypothalamic controls, can in certain cases override or modulate hormonal effects.
hangs from the base of the brain
by a stalk and is enclosed 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 output of the anterior pituitary and synthesizes two hormones that it exports to the posterior pituitary for storage and later release.
Four of the six adenohypophyseal hormones are tropic hormone
s 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 growth of all body tissues but especially skeletal muscle and bone. It may act directly, or indirectly via insulin-like growth factor
s (IGFs). GH mobilizes fats, stimulates protein synthesis, and inhibits glucose uptake 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
(TSH) promotes normal development and activity of the thyroid gland. Thyrotropin-releasing hormone
(TRH) stimulates its release; negative feedback of thyroid hormone inhibits it.
Adrenocorticotropic hormone
(ACTH) stimulates the adrenal cortex
to release corticosteroids. ACTH release is triggered by corticotropin-releasing hormone (CRH) and inhibited by rising glucocorticoid
levels.
The gonadotropin
s—follicle-stimulating hormone
(FSH) and luteinizing hormone
(LH) regulate the functions of the gonads in both sexes. FSH stimulates sex cell production; LH stimulates gonadal hormone production. Gonadotropin levels rise in response to gonadotropin-releasing hormone
(GnRH). Negative feedback of gonadal hormones inhibits gonadotropin release.
Prolactin
(PRL) promotes milk production in humans females. Its secretion is prompted by prolactin-releasing hormone
(PRH) and inhibited by prolactin-inhibiting hormone (PIH).
The neurohypophysis
stores and releases two hypothalamic hormones:
, a glycoprotein from which thyroid hormone is derived.
Thyroid hormone
(TH) includes thyroxine
(T4) and triiodothyronine
(T3), which increase the rate of cellular metabolism
. Consequently, oxygen use and heat production rise.
Secretion of thyroid hormone, prompted by TSH, requires reuptake of the stored colloid by the follicle cells and splitting of the hormones from the colloid for release. Rising levels of thyroid hormone feed back to inhibit the pituitary and hypothalamus.
Most T4 is converted to T3 (the more active form) in the target tissues. These hormones act by turning on gene transcription and protein synthesis.
Graves' disease
is the most common cause of hyperthyroidism
; hyposecretion causes cretinism
in infants and myxoedema in adults.
Calcitonin
, produced by the parafollicular cells of the thyroid gland in response to rising blood calcium levels, depresses blood calcium levels by inhibiting bone matrix resorption and enhancing calcium deposit in bone.
(PTH), which causes an increase in blood calcium levels by targeting bone, the intestine, and the kidneys. PTH is the antagonist of calcitonin
. PTH release is triggered by falling blood calcium levels and is inhibited by rising blood calcium levels.
Hyperparathyroidism
results in hypercalcaemia
and all its effects and in extreme bone wasting. Hypoparathyroidism
leads to hypocalcaemia
, evidenced by tetany and respiratory paralysis.
and an endocrine gland. The endocrine portion (islets of langerhans) releases insulin and glucagon and smaller amounts of other hormones to the blood.
Glucagon
, released by alpha (α) cells when glucose level in blood are low, stimulates the liver to release glucose
to the blood.
Insulin
is released by beta (β) cells when blood levels of glucose (and amino acids) are rising. It increases the rate of glucose uptake and metabolism by most body cells. Hyposecretion of insulin results in diabetes mellitus; cardinal signs are polyuria, polydipsia, and polyphagia.
s by the ovarian follicles begins at puberty
under the influence of FSH. Estrogens stimulate maturation of the female reproductive system and development of the secondary sex characteristics. Progesterone
is released in response to high blood levels of LH. It works with estrogens in establishing the menstrual cycle
.
The testes of the male begin to produce testosterone
at puberty in response to LH. Testosterone promotes maturation of the male reproductive organs, development of secondary sex characteristics, and production of sperm by the testes.
. Its primary hormone is melatonin
, which influences daily rhythms and may have an antigonadotropic effect in humans.
(atrial natriuretic peptide
); gastrointestinal tract
organs (gastrin
, secretin
, and others); the placenta
(hormones of pregnancy—estrogen
, progesterone
, and others); the kidney
s (erythropoietin
and renin
); the thymus
; skin
(cholecalciferol
); and adipose tissue
(leptin
and resistin
).
B
The natural decrease in function of the female’s ovaries during late middle age results in menopause
. The efficiency of all endocrine glands seems to decrease gradually as aging occurs. This leads to a generalized increase in the incidence of diabetes mellitus
and a lower metabolic rate
.
Endocrine system
In physiology, the endocrine system is a system of glands, each of which secretes a type of hormone directly into the bloodstream to regulate the body. The endocrine system is in contrast to the exocrine system, which secretes its chemicals using ducts. It derives from the Greek words "endo"...
that secrete their products, hormone
Hormone
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...
s, directly into the blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
rather than through a duct. The main endocrine glands include the pituitary gland
Pituitary gland
In vertebrate anatomy the pituitary gland, or hypophysis, is an endocrine gland about the size of a pea and weighing 0.5 g , in humans. It is a protrusion off the bottom of the hypothalamus at the base of the brain, and rests in a small, bony cavity covered by a dural fold...
, pancreas
Pancreas
The pancreas is a gland organ in the digestive and endocrine system of vertebrates. It is both an endocrine gland producing several important hormones, including insulin, glucagon, and somatostatin, as well as a digestive organ, secreting pancreatic juice containing digestive enzymes that assist...
, ovaries
Ovary
The ovary is an ovum-producing reproductive organ, often found in pairs as part of the vertebrate female reproductive system. Ovaries in anatomically female individuals are analogous to testes in anatomically male individuals, in that they are both gonads and endocrine glands.-Human anatomy:Ovaries...
, testes
Testicle
The testicle is the male gonad in animals. Like the ovaries to which they are homologous, testes are components of both the reproductive system and the endocrine system...
, thyroid gland
Thyroid
The thyroid gland or simply, the thyroid , in vertebrate anatomy, is one of the largest endocrine glands. The thyroid gland is found in the neck, below the thyroid cartilage...
, and adrenal gland
Adrenal gland
In mammals, the adrenal glands are endocrine glands that sit atop the kidneys; in humans, the right suprarenal gland is triangular shaped, while the left suprarenal gland is semilunar shaped...
s. The hypothalamus
Hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...
is a neuroendocrine organ. Other organ
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...
s which are not so well known for their endocrine activity include the stomach
Stomach
The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...
, which produces such hormones as ghrelin
Ghrelin
Ghrelin is a 28 amino acid peptide and hormone that is produced mainly by P/D1 cells lining the fundus of the human stomach and epsilon cells of the pancreas that stimulates hunger. Ghrelin levels increase before meals and decrease after meals. It is considered the counterpart of the hormone...
.
Local chemical messengers, not generally considered part of the endocrine system, include autocrine
Autocrine signalling
Autocrine signaling is a form of signalling in which a cell secretes a hormone or chemical messenger that binds to autocrine receptors on the same cell, leading to changes in the cell...
s, which act on the cells that secrete them, and paracrine
Paracrine signalling
Paracrine signalling is a form of cell signalling in which the target cell is near the signal-releasing cell.-Local action:Some signalling molecules degrade very quickly, limiting the scope of their effectiveness to the immediate surroundings...
s, which act on a different cell type nearby.
The ability of a target cell
Target cell
-Hematology:In hematology, target cells are very abnormal red blood cells, that have a bullseye appearance . These target cells are associated with Hemoglobin C disease, Asplenia, Liver Disease, Thalassemia and severe Iron deficiency anemia...
to respond to a hormone depends on the presence of receptors, within the cell or on its plasma membrane, to which the hormone can bind.
Hormone receptors are dynamic structures. Changes in number and sensitivity of hormone receptors may occur in response to high or low levels of stimulating hormones.
Blood levels of hormones reflect a balance between secretion
Secretion
Secretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...
and degradation/excretion
Excretion
Excretion is the process by which waste products of metabolism and other non-useful materials are eliminated from an organism. This is primarily carried out by the lungs, kidneys and skin. This is in contrast with secretion, where the substance may have specific tasks after leaving the cell...
. The liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
and kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...
s are the major organs that degrade hormones; breakdown products are excreted in urine and feces.
Hormone half-life and duration of activity are limited and vary from hormone to hormone.
Interaction of hormones at target cells
PermissivenessPermissiveness (biology)
In biology, permissiveness is a certain relationship between hormones and the target cell. It can be applied to describe situations in which the presence of one hormone, at a certain concentration, is required in order to allow a second hormone to fully affect the target cell...
is the situation in which a hormone cannot exert its full effects without the presence of another hormone.
Synergism
Synergism
In theology, synergism is the position of those who hold that salvation involves some form of cooperation between divine grace and human freedom...
occurs when two or more hormones produce the same effects in a target cell and their results are amplified.
Antagonism
Antagonism
Antagonism is hostility that results in active resistance, opposition, or contentiousness.Additionally, it may refer to:*Antagonism , where the involvement of multiple agents reduces their overall effect...
occurs when a hormone opposes or reverses the effect of another hormone.
Control of hormone release
Endocrine organs are activated to release their hormones by humoral, neural, or hormonal stimuli. Negative feedback is important in regulating hormoneHormone
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...
levels in the blood.
The nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
, acting through hypothalamic controls, can in certain cases override or modulate hormonal effects.
Pituitary gland (hypophysis)
The pituitary glandPituitary gland
In vertebrate anatomy the pituitary gland, or hypophysis, is an endocrine gland about the size of a pea and weighing 0.5 g , in humans. It is a protrusion off the bottom of the hypothalamus at the base of the brain, and rests in a small, bony cavity covered by a dural fold...
hangs from the base of the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
by a stalk and is enclosed 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
Hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...
. The hypothalamus regulates the hormonal output of the anterior pituitary and synthesizes two hormones that it exports to the posterior pituitary for storage and later release.
Four of the six adenohypophyseal hormones are tropic hormone
Tropic hormone
Tropic hormones are hormones that have other endocrine glands as their target. Most tropic hormones are produced and secreted by the anterior pituitary...
s 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
Growth hormone
Growth hormone is a peptide hormone that stimulates growth, cell reproduction and regeneration in humans and other animals. Growth hormone is a 191-amino acid, single-chain polypeptide that is synthesized, stored, and secreted by the somatotroph cells within the lateral wings of the anterior...
(GH) is an anabolic hormone
Anabolism
Anabolism is the set of metabolic pathways that construct molecules from smaller units. These reactions require energy. One way of categorizing metabolic processes, whether at the cellular, organ or organism level is as 'anabolic' or as 'catabolic', which is the opposite...
that stimulates growth of all body tissues but especially skeletal muscle and bone. It may act directly, or indirectly via insulin-like growth factor
Insulin-like growth factor
The insulin-like growth factors are proteins with high sequence similarity to insulin. IGFs are part of a complex system that cells use to communicate with their physiologic environment...
s (IGFs). GH mobilizes fats, stimulates protein synthesis, and inhibits glucose uptake and metabolism. Secretion is regulated by growth hormone releasing hormone
Growth hormone releasing hormone
Growth-hormone-releasing hormone , also known as growth-hormone-releasing factor , somatoliberin or somatocrinin, is a releasing hormone for growth hormone...
(GHRH) and growth hormone inhibiting hormone
Somatostatin
Somatostatin is a peptide hormone that regulates the endocrine system and affects neurotransmission and cell proliferation via interaction with G-protein-coupled somatostatin receptors and inhibition of the release of numerous secondary hormones.Somatostatin...
(GHIH), or somatostatin. Hypersecretion causes gigantism
Gigantism
Gigantism, also known as giantism , is a condition characterized by excessive growth and height significantly above average...
in children and acromegaly
Acromegaly
Acromegaly is a syndrome that results when the anterior pituitary gland produces excess growth hormone after epiphyseal plate closure at puberty...
in adults; hyposecretion in children causes pituitary dwarfism
Growth hormone deficiency
Growth hormone deficiency is a medical condition in which the body does not produce enough growth hormone . Growth hormone, also called somatotropin, is a polypeptide hormone which stimulates growth and cell reproduction...
.
Thyroid-stimulating hormone
Thyroid-stimulating hormone
Thyrotrophin-stimulating hormone is a peptide hormone synthesized and secreted by thyrotrope cells in the anterior pituitary gland, which regulates the endocrine function of the thyroid gland.- Physiology :...
(TSH) promotes normal development and activity of the thyroid gland. Thyrotropin-releasing hormone
Thyrotropin-releasing hormone
Thyrotropin-releasing hormone , also called thyrotropin-releasing factor , thyroliberin or protirelin, is a tropic tripeptide hormone that stimulates the release of thyroid-stimulating hormone and prolactin by the anterior pituitary...
(TRH) stimulates its release; negative feedback of thyroid hormone inhibits it.
Adrenocorticotropic hormone
Adrenocorticotropic hormone
Adrenocorticotropic hormone , also known as 'corticotropin', 'Adrenocorticotrophic hormone', is a polypeptide tropic hormone produced and secreted by the anterior pituitary gland. It is an important component of the hypothalamic-pituitary-adrenal axis and is often produced in response to biological...
(ACTH) stimulates the adrenal cortex
Adrenal cortex
Situated along the perimeter of the adrenal gland, the adrenal cortex mediates the stress response through the production of mineralocorticoids and glucocorticoids, including aldosterone and cortisol respectively. It is also a secondary site of androgen synthesis.-Layers:Notably, the reticularis in...
to release corticosteroids. ACTH release is triggered by corticotropin-releasing hormone (CRH) and inhibited by rising glucocorticoid
Glucocorticoid
Glucocorticoids are a class of steroid hormones that bind to the glucocorticoid receptor , which is present in almost every vertebrate animal cell...
levels.
The gonadotropin
Gonadotropin
Gonadotropins are protein hormones secreted by gonadotrope cells of the pituitary gland of vertebrates. This is a family of proteins, which include the mammalian hormones follitropin , lutropin , placental chorionic gonadotropins hCG and eCG and chorionic gonadotropin , as well as at least two...
s—follicle-stimulating hormone
Follicle-stimulating hormone
Follicle-stimulating hormone is a hormone found in humans and other animals. It is synthesized and secreted by gonadotrophs of the anterior pituitary gland. FSH regulates the development, growth, pubertal maturation, and reproductive processes of the body. FSH and Luteinizing hormone act...
(FSH) and luteinizing hormone
Luteinizing hormone
Luteinizing hormone is a hormone produced by the anterior pituitary gland. In females, an acute rise of LH called the LH surge triggers ovulation and development of the corpus luteum. In males, where LH had also been called interstitial cell-stimulating hormone , it stimulates Leydig cell...
(LH) regulate the functions of the gonads in both sexes. FSH stimulates sex cell production; LH stimulates gonadal hormone production. Gonadotropin levels rise in response to gonadotropin-releasing hormone
Gonadotropin-releasing hormone
Gonadotropin-releasing hormone , also known as Luteinizing-hormone-releasing hormone and luliberin, is a tropic peptide hormone responsible for the release of follicle-stimulating hormone and luteinizing hormone from the anterior pituitary. GnRH is synthesized and released from neurons within...
(GnRH). Negative feedback of gonadal hormones inhibits gonadotropin release.
Prolactin
Prolactin
Prolactin also known as luteotropic hormone is a protein that in humans is encoded by the PRL gene.Prolactin is a peptide hormone discovered by Henry Friesen...
(PRL) promotes milk production in humans females. Its secretion is prompted by prolactin-releasing hormone
PRLH
Prolactin releasing hormone, also known as PRLH, is a hypothetical human hormone or hormone releasing factor. Existence of this factor has been hypothesized as prolactin is the only currently known hormone for which almost exclusively negative regulating factors are known but few stimulating factors...
(PRH) and inhibited by prolactin-inhibiting hormone (PIH).
The neurohypophysis
Posterior pituitary
The posterior pituitary comprises the posterior lobe of the pituitary gland and is part of the endocrine system. Despite its name, the posterior pituitary gland is not a gland, per se; rather, it is largely a collection of axonal projections from the hypothalamus that terminate behind the anterior...
stores and releases two hypothalamic hormones:
- OxytocinOxytocinOxytocin is a mammalian hormone that acts primarily as a neuromodulator in the brain.Oxytocin is best known for its roles in sexual reproduction, in particular during and after childbirth...
stimulates powerful uterine contractions, which trigger labor and delivery of an infant, and milk ejection in nursing women. Its release is mediated reflexively by the hypothalamus and represents a positive feedback mechanism. - Antidiuretic hormoneVasopressinArginine vasopressin , also known as vasopressin, argipressin or antidiuretic hormone , is a neurohypophysial hormone found in most mammals, including humans. Vasopressin is a peptide hormone that controls the reabsorption of molecules in the tubules of the kidneys by affecting the tissue's...
(ADH) stimulates the kidney tubules to reabsorb and conserve water, resulting in small volumes of highly concentrated urine and decreased plasma osmolality. ADH is released in response to high solute concentrations in the blood and inhibited by low solute concentrations in the blood. Hyposecretion results in diabetes insipidus.
Thyroid gland
The thyroid gland is located in the anterior throat. Thyroid follicles store colloid containing thyroglobulinThyroglobulin
Thyroglobulin is a 660 kDa, dimeric protein produced by and used entirely within the thyroid gland. In earlier literature, Tg was referred to as colloid....
, a glycoprotein from which thyroid hormone is derived.
Thyroid hormone
Thyroid hormone
The thyroid hormones, thyroxine and triiodothyronine , are tyrosine-based hormones produced by the thyroid gland primarily responsible for regulation of metabolism. An important component in the synthesis of thyroid hormones is iodine. The major form of thyroid hormone in the blood is thyroxine ,...
(TH) includes thyroxine
Thyroxine
Thyroxine, or 3,5,3',5'-tetraiodothyronine , a form of thyroid hormones, is the major hormone secreted by the follicular cells of the thyroid gland.-Synthesis and regulation:...
(T4) and triiodothyronine
Triiodothyronine
Triiodothyronine, C15H12I3NO4, also known as T3, is a thyroid hormone. It affects almost every physiological process in the body, including growth and development, metabolism, body temperature, and heart rate....
(T3), which increase the rate of cellular metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
. Consequently, oxygen use and heat production rise.
Secretion of thyroid hormone, prompted by TSH, requires reuptake of the stored colloid by the follicle cells and splitting of the hormones from the colloid for release. Rising levels of thyroid hormone feed back to inhibit the pituitary and hypothalamus.
Most T4 is converted to T3 (the more active form) in the target tissues. These hormones act by turning on gene transcription and protein synthesis.
Graves' disease
Graves' disease
Graves' disease is an autoimmune disease where the thyroid is overactive, producing an excessive amount of thyroid hormones...
is the most common cause of hyperthyroidism
Hyperthyroidism
Hyperthyroidism is the term for overactive tissue within the thyroid gland causing an overproduction of thyroid hormones . Hyperthyroidism is thus a cause of thyrotoxicosis, the clinical condition of increased thyroid hormones in the blood. Hyperthyroidism and thyrotoxicosis are not synonymous...
; hyposecretion causes cretinism
Cretinism
Cretinism is a condition of severely stunted physical and mental growth due to untreated congenital deficiency of thyroid hormones usually due to maternal hypothyroidism.-Etymology and use of cretin:...
in infants and myxoedema in adults.
Calcitonin
Calcitonin
Calcitonin is a 32-amino acid linear polypeptide hormone that is producedin humans primarily by the parafollicular cells of the thyroid, and in many other animals in the ultimobranchial body. It acts to reduce blood calcium , opposing the effects of parathyroid hormone . Calcitonin has been found...
, produced by the parafollicular cells of the thyroid gland in response to rising blood calcium levels, depresses blood calcium levels by inhibiting bone matrix resorption and enhancing calcium deposit in bone.
Parathyroid glands
The parathyroid glands, located on the dorsal aspect of the thyroid gland, secrete parathyroid hormoneParathyroid hormone
Parathyroid hormone , parathormone or parathyrin, is secreted by the chief cells of the parathyroid glands as a polypeptide containing 84 amino acids...
(PTH), which causes an increase in blood calcium levels by targeting bone, the intestine, and the kidneys. PTH is the antagonist of calcitonin
Calcitonin
Calcitonin is a 32-amino acid linear polypeptide hormone that is producedin humans primarily by the parafollicular cells of the thyroid, and in many other animals in the ultimobranchial body. It acts to reduce blood calcium , opposing the effects of parathyroid hormone . Calcitonin has been found...
. PTH release is triggered by falling blood calcium levels and is inhibited by rising blood calcium levels.
Hyperparathyroidism
Hyperparathyroidism
Hyperparathyroidism is overactivity of the parathyroid glands resulting in excess production of parathyroid hormone . The parathyroid hormone regulates calcium and phosphate levels and helps to maintain these levels...
results in hypercalcaemia
Hypercalcaemia
Hypercalcaemia is an elevated calcium level in the blood. . It can be an asymptomatic laboratory finding, but because an elevated calcium level is often indicative of other diseases, a workup should be undertaken if it persists...
and all its effects and in extreme bone wasting. Hypoparathyroidism
Hypoparathyroidism
Hypoparathyroidism is decreased function of the parathyroid glands with under production of parathyroid hormone. This can lead to low levels of calcium in the blood, often causing cramping and twitching of muscles or tetany , and several other symptoms...
leads to hypocalcaemia
Hypocalcaemia
In medicine, hypocalcaemia is the presence of low serum calcium levels in the blood, usually taken as less than 2.1 mmol/L or 9 mg/dl or an ionized calcium level of less than 1.1 mmol/L or 4.5 mg/dL. It is a type of electrolyte disturbance...
, evidenced by tetany and respiratory paralysis.
Pancreas
The pancreas, located in the abdomen close to the stomach, is both an exocrineExocrine gland
Exocrine glands are a type of ductal glands that secrete their products into ducts that lead directly into the external environment...
and an endocrine gland. The endocrine portion (islets of langerhans) releases insulin and glucagon and smaller amounts of other hormones to the blood.
Glucagon
Glucagon
Glucagon, a hormone secreted by the pancreas, raises blood glucose levels. Its effect is opposite that of insulin, which lowers blood glucose levels. The pancreas releases glucagon when blood sugar levels fall too low. Glucagon causes the liver to convert stored glycogen into glucose, which is...
, released by alpha (α) cells when glucose level in blood are low, stimulates the liver to release glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
to the blood.
Insulin
Insulin
Insulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....
is released by beta (β) cells when blood levels of glucose (and amino acids) are rising. It increases the rate of glucose uptake and metabolism by most body cells. Hyposecretion of insulin results in diabetes mellitus; cardinal signs are polyuria, polydipsia, and polyphagia.
Gonads
The ovaries of the female, located in the pelvic cavity, release two main hormones. Secretion of estrogenEstrogen
Estrogens , oestrogens , or œstrogens, are a group of compounds named for their importance in the estrous cycle of humans and other animals. They are the primary female sex hormones. Natural estrogens are steroid hormones, while some synthetic ones are non-steroidal...
s by the ovarian follicles begins at puberty
Puberty
Puberty is the process of physical changes by which a child's body matures into an adult body capable of reproduction, as initiated by hormonal signals from the brain to the gonads; the ovaries in a girl, the testes in a boy...
under the influence of FSH. Estrogens stimulate maturation of the female reproductive system and development of the secondary sex characteristics. Progesterone
Progesterone
Progesterone also known as P4 is a C-21 steroid hormone involved in the female menstrual cycle, pregnancy and embryogenesis of humans and other species...
is released in response to high blood levels of LH. It works with estrogens in establishing the menstrual cycle
Menstrual cycle
The menstrual cycle is the scientific term for the physiological changes that can occur in fertile women for the purpose of sexual reproduction. This article focuses on the human menstrual cycle....
.
The testes of the male begin to produce testosterone
Testosterone
Testosterone is a steroid hormone from the androgen group and is found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is primarily secreted in the testes of males and the ovaries of females, although small amounts are also secreted by the adrenal glands...
at puberty in response to LH. Testosterone promotes maturation of the male reproductive organs, development of secondary sex characteristics, and production of sperm by the testes.
Pineal gland
The pineal gland is located in the diencephalonDiencephalon
The diencephalon is the region of the vertebrate neural tube which gives rise to posterior forebrain structures. In development, the forebrain develops from the prosencephalon, the most anterior vesicle of the neural tube which later forms both the diencephalon and the...
. Its primary hormone is melatonin
Melatonin
Melatonin , also known chemically as N-acetyl-5-methoxytryptamine, is a naturally occurring compound found in animals, plants, and microbes...
, which influences daily rhythms and may have an antigonadotropic effect in humans.
Other hormone-producing structures
Many body organs not normally considered endocrine organs contain isolated cell clusters that secrete hormones. Examples include the heartHeart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
(atrial natriuretic peptide
Atrial natriuretic peptide
Atrial natriuretic peptide , atrial natriuretic factor , atrial natriuretic hormone , or atriopeptin, is a powerful vasodilator, and a protein hormone secreted by heart muscle cells. It is involved in the homeostatic control of body water, sodium, potassium and fat...
); gastrointestinal tract
Gastrointestinal tract
The human gastrointestinal tract refers to the stomach and intestine, and sometimes to all the structures from the mouth to the anus. ....
organs (gastrin
Gastrin
In humans, gastrin is a peptide hormone that stimulates secretion of gastric acid by the parietal cells of the stomach and aids in gastric motility. It is released by G cells in the antrum of the stomach, duodenum, and the pancreas...
, secretin
Secretin
Secretin is a hormone that controls the secretions into the duodenum, and also separately, water homeostasis throughout the body. It is produced in the S cells of the duodenum in the crypts of Lieberkühn...
, and others); the placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
(hormones of pregnancy—estrogen
Estrogen
Estrogens , oestrogens , or œstrogens, are a group of compounds named for their importance in the estrous cycle of humans and other animals. They are the primary female sex hormones. Natural estrogens are steroid hormones, while some synthetic ones are non-steroidal...
, progesterone
Progesterone
Progesterone also known as P4 is a C-21 steroid hormone involved in the female menstrual cycle, pregnancy and embryogenesis of humans and other species...
, and others); the kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...
s (erythropoietin
Erythropoietin
Erythropoietin, or its alternatives erythropoetin or erthropoyetin or EPO, is a glycoprotein hormone that controls erythropoiesis, or red blood cell production...
and renin
Renin
Renin , also known as an angiotensinogenase, is an enzyme that participates in the body's renin-angiotensin system -- also known as the Renin-Angiotensin-Aldosterone Axis -- that mediates extracellular volume , and arterial vasoconstriction...
); the thymus
Thymus
The thymus is a specialized organ of the immune system. The thymus produces and "educates" T-lymphocytes , which are critical cells of the adaptive immune system....
; skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
(cholecalciferol
Cholecalciferol
Cholecalciferol is a form of vitamin D, also called vitamin D3 or calciol.It is structurally similar to steroids such as testosterone, cholesterol, and cortisol .-Forms:Vitamin D3 has several forms:...
); and adipose tissue
Adipose tissue
In histology, adipose tissue or body fat or fat depot or just fat is loose connective tissue composed of adipocytes. It is technically composed of roughly only 80% fat; fat in its solitary state exists in the liver and muscles. Adipose tissue is derived from lipoblasts...
(leptin
Leptin
Leptin is a 16 kDa protein hormone that plays a key role in regulating energy intake and energy expenditure, including appetite and metabolism. It is one of the most important adipose derived hormones...
and resistin
Resistin
Resistin also known as adipose tissue-specific secretory factor or C/EBP-epsilon-regulated myeloid-specific secreted cysteine-rich protein is a cysteine-rich protein that in humans is encoded by the RETN gene....
).
B
Developmental aspects of the endocrine system
Endocrine glands derive from all three germ layers. Those derived from mesoderm produce steroidal hormones; the others produce the amino acid–based hormones.The natural decrease in function of the female’s ovaries during late middle age results in menopause
Menopause
Menopause is a term used to describe the permanent cessation of the primary functions of the human ovaries: the ripening and release of ova and the release of hormones that cause both the creation of the uterine lining and the subsequent shedding of the uterine lining...
. The efficiency of all endocrine glands seems to decrease gradually as aging occurs. This leads to a generalized increase in the incidence of diabetes mellitus
Diabetes mellitus
Diabetes mellitus, often simply referred to as diabetes, is a group of metabolic diseases in which a person has high blood sugar, either because the body does not produce enough insulin, or because cells do not respond to the insulin that is produced...
and a lower metabolic rate
Basal metabolic rate
Basal Metabolic Rate , and the closely related resting metabolic rate , is the amount of daily energy expended by humans and other animals at rest. Rest is defined as existing in a neutrally temperate environment while in the post-absorptive state...
.