Thyroid Gland: Location and Structure

The largest pure endocrine gland in the body, The largest pure endocrine gland in the body, located in the front of the neck, on the trachea just located in the front of the neck, on the trachea just below to the larynx.below to the larynx.

Its two lobes are connected by a median tissue Its two lobes are connected by a median tissue mass called the isthmus. mass called the isthmus.

Internally, it is composed of about 1 million of Internally, it is composed of about 1 million of round round folliclesfollicles. The walls of each follice are . The walls of each follice are formed by cuboidal and squamous epithelial cells formed by cuboidal and squamous epithelial cells called follicle cells, which produce called follicle cells, which produce thyroglobulinthyroglobulin (glycoprotein). (glycoprotein).

The lumen of each follicle stores The lumen of each follicle stores colloidcolloid, which , which consists primarily of molecules of thyroglobulin.consists primarily of molecules of thyroglobulin.

The follicular epithelium also consists of The follicular epithelium also consists of parafollicular cells, a separate population of parafollicular cells, a separate population of endocrine cells that produce calcitonin, a hormone endocrine cells that produce calcitonin, a hormone involved in calcium homeostasis.involved in calcium homeostasis.

Thyroid hormones (THs)Thyroid hormones (THs) The two THs contain iodine and are called thyroxin or TThe two THs contain iodine and are called thyroxin or T44 and and

triiodothyronine or Ttriiodothyronine or T33. .

TT44 and T and T33 have a very similar structure as each is made up of two have a very similar structure as each is made up of two tyrosine amino acids linked together and either 4 or 3 atoms of iodine, tyrosine amino acids linked together and either 4 or 3 atoms of iodine, respectively.respectively.

TT44 is the main hormone produced by the thyroid and T is the main hormone produced by the thyroid and T33 has most if not has most if not all of biological activity as all target tissues rapidly convert Tall of biological activity as all target tissues rapidly convert T44 to T to T33..

Except for the adult brain, spleen, testes, and the thyroid gland itself, Except for the adult brain, spleen, testes, and the thyroid gland itself, THs affect all other types of cells in the body where they stimulate THs affect all other types of cells in the body where they stimulate activity of enzymes especially those involved in glucose metabolismactivity of enzymes especially those involved in glucose metabolism

Increase metabolic rate in target tissues, which increases body heat Increase metabolic rate in target tissues, which increases body heat production (calorigenic effect).production (calorigenic effect).

THs also are critically important for normal growth and development THs also are critically important for normal growth and development of skeletal and nervous systems and maturation of reproductive of skeletal and nervous systems and maturation of reproductive system.system.

Synthesis of thyroid Synthesis of thyroid hormones:hormones:

Formation and storage of thyroglobulin. Formation and storage of thyroglobulin.

This process takes place in follicle cells and the final This process takes place in follicle cells and the final product is packed into vesicles, their contents are product is packed into vesicles, their contents are discharged into the lumen of the follicle and become a discharged into the lumen of the follicle and become a major part of the colloid.major part of the colloid.

Iodide trapping and oxidation to iodine. Iodide trapping and oxidation to iodine.

To produce functional iodinated hormones, follicle cells To produce functional iodinated hormones, follicle cells accumulate iodide from the blood. A protein pump accumulate iodide from the blood. A protein pump (iodide trap), located on the basal surface of follicle (iodide trap), located on the basal surface of follicle cells, actively transports iodide into follicle cells where cells, actively transports iodide into follicle cells where it is oxidized and converted to iodine (Iit is oxidized and converted to iodine (I22).).

Iodination. Iodination.

Once formed, iodine is attached to tyrosine Once formed, iodine is attached to tyrosine amino acids which are part of the thyroglobulin.amino acids which are part of the thyroglobulin.

Coupling. Coupling.

Iodination of one tyrosine produces Iodination of one tyrosine produces monoiodotyrosine (MIT), iodination of two monoiodotyrosine (MIT), iodination of two tyrosines diiodotyrosine (DIT). tyrosines diiodotyrosine (DIT).

Then enzymes within the colloid link MITs and Then enzymes within the colloid link MITs and DITs in a highly specific fashion, as a result two DITs in a highly specific fashion, as a result two DITs linked together result in T4 , while coupling DITs linked together result in T4 , while coupling of MIT and DIT produce T3. of MIT and DIT produce T3.

Interactions between two DITs are more Interactions between two DITs are more frequent so more thyroxin. frequent so more thyroxin.

At this point both thyroid hormones are still At this point both thyroid hormones are still attached to thyroglobulin molecules in the attached to thyroglobulin molecules in the colloid.colloid.

Colloid endocytosis. Colloid endocytosis.

Colloid droplets containing iodinated thyroglobulin Colloid droplets containing iodinated thyroglobulin are taken up by follicle cells by endocytosis. These are taken up by follicle cells by endocytosis. These combine with lysosomes to form phagolysosomes.combine with lysosomes to form phagolysosomes.

Cleavage of the hormones for release. Cleavage of the hormones for release.

Within the phagolysosomes, the hormones are Within the phagolysosomes, the hormones are cleaved from the thyroglobulin by lysosomal cleaved from the thyroglobulin by lysosomal enzymes. The free hormones then diffuse through enzymes. The free hormones then diffuse through the basal membrane out of the follicle cell and into the basal membrane out of the follicle cell and into the blood stream.the blood stream.

Transport and regulation of Transport and regulation of release:release:

Most released TMost released T44 and T and T33 immediately bind to immediately bind to plasma proteins, of which the most important is plasma proteins, of which the most important is thyroxin-binding globulin (TBG) produced by the thyroxin-binding globulin (TBG) produced by the liver. liver.

Binding proteins protect TBinding proteins protect T44 and T and T33 from immediate from immediate degeneration by plasma enzymes, also they allow degeneration by plasma enzymes, also they allow TT44 and T and T33 to reach target tissues, often located a to reach target tissues, often located a significant distance away from the thyroid gland. significant distance away from the thyroid gland.

Decreasing blood levels of thyroxin trigger release Decreasing blood levels of thyroxin trigger release of TSH from the anterior pituitary, which of TSH from the anterior pituitary, which stimulates the thyroid gland to produce more stimulates the thyroid gland to produce more thyroxin.thyroxin.

Pathology of the thyroid gland Pathology of the thyroid gland function:function:

Both hypo- and hyperactivity and of the thyroid Both hypo- and hyperactivity and of the thyroid gland can cause severe metabolic disturbances. gland can cause severe metabolic disturbances.

In adults, hypothyroidism is referred to asIn adults, hypothyroidism is referred to as myxedema. myxedema.

Symptoms: Symptoms:

Low metabolic rate, poor resistance to cold Low metabolic rate, poor resistance to cold temperatures, constipation, dry skin (especially facial), temperatures, constipation, dry skin (especially facial), puffy eyes, lethargy and mental sluggishness. puffy eyes, lethargy and mental sluggishness.

If hypothyroidism results from lack of iodine the If hypothyroidism results from lack of iodine the thyroid gland enlarges to form a thyroid gland enlarges to form a goitergoiter..

Severe hypothyroidism during the fetal development and in infants is called cretinism.

Symptoms: A short disproportionate body, a thick

tongue and neck, and mental retardation. The condition is preventable by thyroid

hormone replacement therapy. However, once developmental abnormalities and mental retardation appear, they are not reversible.

Hyperthyroidism:Hyperthyroidism:

The most common form of hyperthyroidism is Grave's disease, The most common form of hyperthyroidism is Grave's disease, believed to be an autoimmune disease. believed to be an autoimmune disease.

The immune system produces antibodies that mimic TSH, The immune system produces antibodies that mimic TSH, which bind to TSH receptors and permanently switch them on, which bind to TSH receptors and permanently switch them on, resulting in continuous release of thyroid hormones. resulting in continuous release of thyroid hormones.

Typical symptoms include metabolic rate, sweating, rapid and Typical symptoms include metabolic rate, sweating, rapid and irregular heartbeat, nervousness, and weight loss despite irregular heartbeat, nervousness, and weight loss despite adequate food intake. adequate food intake.

Often, exophthalmos, or protrusion of the eyeballs, occurs Often, exophthalmos, or protrusion of the eyeballs, occurs caused by the edema of tissues behind the eyes followed by caused by the edema of tissues behind the eyes followed by fibrosis. fibrosis.

Treatments include surgical removal of the thyroid gland (very Treatments include surgical removal of the thyroid gland (very difficult due to an extremely rich blood supply) or ingestion of difficult due to an extremely rich blood supply) or ingestion of radioactive iodine (radioactive iodine (131131I), which selectively destroys the most I), which selectively destroys the most active thyroid cells.active thyroid cells.

Hyperthyroidism and Grave’s Hyperthyroidism and Grave’s DiseaseDisease

Parathyroid Glands:Parathyroid Glands:

The parathyroid The parathyroid glands are small in glands are small in size and are found size and are found on the posterior on the posterior aspect of the thyroid aspect of the thyroid gland. gland.

Typically, there are Typically, there are four of them but the four of them but the actual number may actual number may vary. vary.

Histology of the Histology of the ParathyroidParathyroid

The endocrine The endocrine cells within these cells within these glands are glands are arranged in thick, arranged in thick, branching cords branching cords containing containing oxyphil cells of oxyphil cells of unclear function unclear function and most and most importantly large importantly large numbers of numbers of chief chief cellscells that secrete that secrete parathyroid parathyroid hormone (PTH)hormone (PTH)..

PTH:PTH: Small proteinSmall protein

Single most important hormone controlling Single most important hormone controlling calcium homeostasis. Its release is calcium homeostasis. Its release is triggered by falling blood calcium levels triggered by falling blood calcium levels and inhibited by hypercalcemia (high blood and inhibited by hypercalcemia (high blood calcium). calcium).

There are three target organs for PTH:There are three target organs for PTH:

skeletonskeleton kidneyskidneys intestineintestine

PTH stimulates the PTH stimulates the following on these target following on these target

organs:organs: Osteoclasts (bone absorbing cells) are stimulated to Osteoclasts (bone absorbing cells) are stimulated to

digest bone and release ionic calcium and phosphates digest bone and release ionic calcium and phosphates to the blood.to the blood.

Kidneys are stimulated to reabsorb calcium and Kidneys are stimulated to reabsorb calcium and excrete phosphate.excrete phosphate.

Intestines are stimulated to increase calcium Intestines are stimulated to increase calcium absorption. absorption.

Vitamin D is required for absorption of calcium from Vitamin D is required for absorption of calcium from ingested food. ingested food. For vitamin D to exert this effect, it must first be converted For vitamin D to exert this effect, it must first be converted

by the kidneys to its active formby the kidneys to its active form It is this conversion that is directly stimulated by PTH.It is this conversion that is directly stimulated by PTH.

Pathology of the Pathology of the parathyroid glands:parathyroid glands:

Because calcium is essential for so Because calcium is essential for so many functions, including transmission many functions, including transmission of action potentials, muscle of action potentials, muscle contraction, pacemaker activity in the contraction, pacemaker activity in the heart, and blood clotting, precise heart, and blood clotting, precise control of ionic calcium levels in body control of ionic calcium levels in body fluids is absolutely critical. As a result fluids is absolutely critical. As a result both hyper- and hypoparathyroidism both hyper- and hypoparathyroidism can have severe consequences.can have severe consequences.

Hyperparathyroidism:Hyperparathyroidism: Rare, usually the result of a parathyroid gland tumor.Rare, usually the result of a parathyroid gland tumor.

Results in severe loss of calcium from the bones. Results in severe loss of calcium from the bones.

The bones soften and deform as their mineral salts The bones soften and deform as their mineral salts are replaced by fibrous connective tissue. are replaced by fibrous connective tissue.

Results in hypercalcemia Results in hypercalcemia

Leads to, depression of the nervous system leading to Leads to, depression of the nervous system leading to abnormal reflexes and weakness of the skeletal muscles, and abnormal reflexes and weakness of the skeletal muscles, and formation of kidney stones as excess calcium salts are formation of kidney stones as excess calcium salts are deposited in kidney tubules.deposited in kidney tubules.

Hypoparathyroidism:Hypoparathyroidism:

It is a PTH deficiency, which is a common It is a PTH deficiency, which is a common consequence of parathyroid trauma or consequence of parathyroid trauma or removal during thyroid surgery. removal during thyroid surgery.

The resulting hypocalcemia increases The resulting hypocalcemia increases excitability of neurons and may lead to excitability of neurons and may lead to tetany resulting in uncontrollable muscle tetany resulting in uncontrollable muscle twitches and convulsions, which if twitches and convulsions, which if untreated may progress to spasms of the untreated may progress to spasms of the larynx, respiratory paralysis and death.larynx, respiratory paralysis and death.

ADRENAL GLANDS:ADRENAL GLANDS:

The two adrenal The two adrenal glands are pyramid-glands are pyramid-shaped organs shaped organs found atop the found atop the kidneys.kidneys.

Each gland is Each gland is structurally and structurally and functionally two functionally two endocrine glands in endocrine glands in one.one.

The inner adrenal medulla is made up of nervous tissue and acts as part of the sympathetic nervous system. The outer adrenal cortex forms the bulk (about 80%) of the gland. Each of these regions produces its own set of hormones.

Adrenal Medulla:Adrenal Medulla: It is made up of chromaffin cells which secrete the It is made up of chromaffin cells which secrete the

catecholamines epinephrine (E) (adrenaline) and catecholamines epinephrine (E) (adrenaline) and norepinephrine (NE) (noradrenaline) into the blood. norepinephrine (NE) (noradrenaline) into the blood.

During the fight-or-flight responses, the sympathetic During the fight-or-flight responses, the sympathetic nervous system is activated, including the chromaffin nervous system is activated, including the chromaffin tissue and large amounts of catecholamines (80% of which tissue and large amounts of catecholamines (80% of which is E) are released.is E) are released.

In most cases the two hormones have very similar effects In most cases the two hormones have very similar effects on their target organs. However, E is the more potent on their target organs. However, E is the more potent stimulator of the heart rate and strength of contraction, stimulator of the heart rate and strength of contraction, and metabolic activities, such as breakdown of glycogen and metabolic activities, such as breakdown of glycogen and release of glucose).and release of glucose).

NE has great effect on peripheral vasoconstriction and NE has great effect on peripheral vasoconstriction and blood pressure.blood pressure.

Adrenal Cortex:Adrenal Cortex:

The cells of the adrenal cortex are The cells of the adrenal cortex are arranged in three distinct zones, each zone arranged in three distinct zones, each zone producing corticosteroids.producing corticosteroids.

The The Zona glomerulosaZona glomerulosa is the outer-most is the outer-most layer of cells and it produces layer of cells and it produces mineralocorticoids, that help control the mineralocorticoids, that help control the balance of minerals and water in the blood. balance of minerals and water in the blood.

The The zona fasciculatazona fasciculata is composed of cells is composed of cells that secrete glucocorticoids. that secrete glucocorticoids.

The The zona reticulariszona reticularis produce small produce small amounts of adrenal sex steroids.amounts of adrenal sex steroids.

Hormones of the Adrenal Hormones of the Adrenal CortexCortex MineralocorticoidsMineralocorticoids

Although there are several Although there are several mineralocorticoids, mineralocorticoids, aldosteronealdosterone is by far the is by far the most potent and accounts for more than 95% most potent and accounts for more than 95% of production. Its main function is to maintain of production. Its main function is to maintain sodium balance by reducing excretion of this sodium balance by reducing excretion of this ion from the body. ion from the body.

The primary target organs of aldosterone are The primary target organs of aldosterone are kidney tubules where it stimulates kidney tubules where it stimulates reabsorption of sodium ions from urine back reabsorption of sodium ions from urine back to the bloodstream. to the bloodstream.

Aldosterone also enhances sodium absorption Aldosterone also enhances sodium absorption from sweat, saliva, and gastric juice. from sweat, saliva, and gastric juice.

Secretion of aldosterone is induced by a Secretion of aldosterone is induced by a number of factors such as high blood levels number of factors such as high blood levels of potassium, low blood levels of sodium, and of potassium, low blood levels of sodium, and decreasing blood volume and pressure. decreasing blood volume and pressure.

The reverse conditions inhibit secretion of The reverse conditions inhibit secretion of aldosterone. aldosterone.

Glucocorticoids:Glucocorticoids:

Glucocorticoids influence metabolism of most Glucocorticoids influence metabolism of most body cells, help us resist stress, and are body cells, help us resist stress, and are considered to be absolutely essential to life. considered to be absolutely essential to life.

The most important glucocorticoid in humans is The most important glucocorticoid in humans is cortisolcortisol, , but small amounts of cortisone and corticosterone are but small amounts of cortisone and corticosterone are also produced.also produced.

The main effect of cortisol is to promote gluconeogenesis The main effect of cortisol is to promote gluconeogenesis or formation of glucose from noncarbohydrate or formation of glucose from noncarbohydrate molecules, especially fats and proteins. molecules, especially fats and proteins.

Cortisol also breaks down adipose (fat) tissue, released Cortisol also breaks down adipose (fat) tissue, released fatty acids can be then used by many tissued as a source fatty acids can be then used by many tissued as a source of energy and "saving" glucose for the brain. of energy and "saving" glucose for the brain.

Blood levels of glucocorticoids increase significantly Blood levels of glucocorticoids increase significantly during stress, which helps the body to negotiate the during stress, which helps the body to negotiate the crisis.crisis.

Interestingly, chronic excess of cortisol has significant Interestingly, chronic excess of cortisol has significant anti-inflammatory and anti-immune effects and anti-inflammatory and anti-immune effects and glucocorticoid drugs are often used to control symptoms glucocorticoid drugs are often used to control symptoms of many chronic inflammatory disorders, such as of many chronic inflammatory disorders, such as rheumatoid arthritis or allergic responses.rheumatoid arthritis or allergic responses.

Regulation of glucocorticoid Regulation of glucocorticoid secretion:secretion:

It is provided by a typical negative feedback It is provided by a typical negative feedback system:system:

increased (hypothalamus) CRH increased (hypothalamus) CRH negativenegative

increased (adenohypophysis) ACTH increased (adenohypophysis) ACTH increased (adrenal cortex) cortisolincreased (adrenal cortex) cortisol

Gonadocorticoids (Sex Hormones)Gonadocorticoids (Sex Hormones)

The amount of sex steroids produced by The amount of sex steroids produced by zona reticularis is insignificant compared zona reticularis is insignificant compared to the amounts secreted by the gonads. to the amounts secreted by the gonads.

These hormones may contribute to the These hormones may contribute to the onset of puberty and the appearance of onset of puberty and the appearance of axillary and pubic hair in both males and axillary and pubic hair in both males and females. females.

In adult women adrenal In adult women adrenal androgens androgens (male (male sex hormones, especially testosterone) may sex hormones, especially testosterone) may be, at least partially, responsible for the sex be, at least partially, responsible for the sex drive.drive.

Pathology of the adrenal cortex Pathology of the adrenal cortex functionfunction:

Hyperadrenalism :Hyperadrenalism : It is referred to as Cushing's disease and can be It is referred to as Cushing's disease and can be

caused by a cortisol-secreting tumor in the caused by a cortisol-secreting tumor in the adrenal glands, ACTH-secreting tumor of the adrenal glands, ACTH-secreting tumor of the pituitary, or ACTH secreted by abdominal pituitary, or ACTH secreted by abdominal carcinoma. carcinoma.

However, it most often results from the clinical However, it most often results from the clinical administration of pharmacological (very high) administration of pharmacological (very high) doses of glucocorticoid drugs. doses of glucocorticoid drugs.

The symptoms include a persistent hyperglycemia, The symptoms include a persistent hyperglycemia, dramatic loss of muscle and bone proteins, and dramatic loss of muscle and bone proteins, and water and salt retention, leading to hypertension water and salt retention, leading to hypertension and edema - one of its signs is a swollen "moon" and edema - one of its signs is a swollen "moon" face. The only treatment is a surgical removal of face. The only treatment is a surgical removal of tumor or discontinuation of the drug.tumor or discontinuation of the drug.

Hypoadrenalism :Hypoadrenalism :

It is referred to as Addison's disease It is referred to as Addison's disease and involves significant reduction in and involves significant reduction in plasma glucose and sodium, very high plasma glucose and sodium, very high levels of potassium and loss of weight. levels of potassium and loss of weight. The usual treatment is corticosteroid The usual treatment is corticosteroid replacement therapy.replacement therapy.

THE ENDOCRINE THE ENDOCRINE PANCREAS:PANCREAS:

Located partially behind the stomach, Located partially behind the stomach, the pancreas is a mixed gland composed the pancreas is a mixed gland composed of both endocrine and exocrine cells. of both endocrine and exocrine cells.

More than 98% of the gland is made up More than 98% of the gland is made up of acinar cells producing an enzyme-rich of acinar cells producing an enzyme-rich juice that enters a system of ducts and juice that enters a system of ducts and is delivered to the duodenum of the is delivered to the duodenum of the small intestine during food digestion. small intestine during food digestion.

The remaining 1-2% of cells form about The remaining 1-2% of cells form about 1 million of islets of Langerhans, tiny 1 million of islets of Langerhans, tiny cell clusters that produce pancreatic cell clusters that produce pancreatic hormones. hormones.

The islets have four distinct populations The islets have four distinct populations of cells, the two most important ones are of cells, the two most important ones are alpha cells that produce hormone alpha cells that produce hormone glucagon, and more numerous beta cells glucagon, and more numerous beta cells that synthesize insulin. In addition, delta that synthesize insulin. In addition, delta cells produce somatostatin and F cells cells produce somatostatin and F cells secrete pancreatic polypeptide (PP). secrete pancreatic polypeptide (PP).

Hormones of the Hormones of the Pancreas:Pancreas: Glucagon and insulin are directly responsible for the Glucagon and insulin are directly responsible for the

regulation of blood glucose levels and their effects regulation of blood glucose levels and their effects are exactly opposite: are exactly opposite:

insulin is hypoglycemic (it decreases blood glucose)insulin is hypoglycemic (it decreases blood glucose)

glucagon is hyperglycemic (it increases blood glucagon is hyperglycemic (it increases blood glucose). glucose).

Pancreatic somatostatin inhibits the release of both Pancreatic somatostatin inhibits the release of both insulin and glucagon and slows the activity of the insulin and glucagon and slows the activity of the digestive tract.digestive tract.

PP regulates secretion of pancreatic digestive PP regulates secretion of pancreatic digestive enzymes and inhibits release of bile by the enzymes and inhibits release of bile by the gallbladder.gallbladder.

Glucagon:Glucagon: Glucagon is a 29 amino acid polypeptide with extremely potent Glucagon is a 29 amino acid polypeptide with extremely potent

hyperglycemic properties. One molecule of this hormone can hyperglycemic properties. One molecule of this hormone can induce the release of 100 million molecules of glucose into the induce the release of 100 million molecules of glucose into the blood. blood.

The major target organ of glucagon is the liver, where it promotes:The major target organ of glucagon is the liver, where it promotes:

Breakdown of glycogen to glucose (glycogenolysis)Breakdown of glycogen to glucose (glycogenolysis) Synthesis of glucose from lactic acid and from noncarbohydrate Synthesis of glucose from lactic acid and from noncarbohydrate

molecules such as fatty acids and amino acids (referred to molecules such as fatty acids and amino acids (referred to asgluconeogenesis).asgluconeogenesis).

Release of glucose into the blood by the liverRelease of glucose into the blood by the liver

All these effects € blood sugar levels.All these effects € blood sugar levels.

Secretion of glucagon from the alpha cells is induced by, most Secretion of glucagon from the alpha cells is induced by, most importantly, low blood sugar levels but also by high amino acid levels importantly, low blood sugar levels but also by high amino acid levels in the blood (e.g. following a protein-rich meal). Rising blood sugar in the blood (e.g. following a protein-rich meal). Rising blood sugar concentration and somatostatin from the delta cells inhibit glucagon concentration and somatostatin from the delta cells inhibit glucagon release.release.

Insulin:Insulin: Insulin is a 51 amino acid protein consisting of two Insulin is a 51 amino acid protein consisting of two

polypeptide chains linked by disulfide bonds. It is polypeptide chains linked by disulfide bonds. It is synthesized as part of a larger molecule called proinsulin synthesized as part of a larger molecule called proinsulin and packed into secretory vesicles where its middle and packed into secretory vesicles where its middle portion is excised by enzymes to produce functional portion is excised by enzymes to produce functional hormone, just before insulin is released from the beta cell. hormone, just before insulin is released from the beta cell.

As mentioned earlier, insulin's main function is to lower As mentioned earlier, insulin's main function is to lower blood sugar levels but it also affects protein and fat blood sugar levels but it also affects protein and fat metabolism. metabolism.

In general, insulin:In general, insulin:

Increases membrane transport of glucose into body cells, Increases membrane transport of glucose into body cells, especially muscle and liver cellsespecially muscle and liver cells

Inhibits the breakdown of glycogen (it should not be confused Inhibits the breakdown of glycogen (it should not be confused with glucagon!) into glucose,with glucagon!) into glucose,

Increases the rate of ATP production from glucoseIncreases the rate of ATP production from glucose Increases the rate of glycogen synthesisIncreases the rate of glycogen synthesis Increases the rate of glucose conversion to fat.Increases the rate of glucose conversion to fat.

Insulin binds to tyrosine kinase receptors, but Insulin binds to tyrosine kinase receptors, but mechanism of action, including type(s) and mechanism of action, including type(s) and specific roles of second messengers, are poorly specific roles of second messengers, are poorly understood. understood.

The beta cells are stimulated to produce insulin The beta cells are stimulated to produce insulin primarily by elevated blood sugar levels, but also primarily by elevated blood sugar levels, but also by high blood levels of amino acids and fatty by high blood levels of amino acids and fatty acids. acids.

Several hormones also induce the release of Several hormones also induce the release of insulin, including glucagon, epinephrine, growth insulin, including glucagon, epinephrine, growth hormone, thyroid hormones, and glucocorticoids. hormone, thyroid hormones, and glucocorticoids.

In contrast, somatostatin inhibits insulin release.In contrast, somatostatin inhibits insulin release.