Endocrine System Introduction

8/3/2019 Endocrine System Introduction

1/17


2/17


3/17


4/17


5/17

Endocrine System Introduction

The endocrine system is made up of glands that produce and secrete hormones.

These hormones regulate the body's growth, metabolism (the physical and chemical

processes of the body), and sexual development and function. The hormones are

released into the bloodstream and may affect one or several organs throughout the

body.

Hormones are chemical messengers created by the body. They transfer information

from one set of cells to another to coordinate the functions of different parts of the

body.

Each hormone has target cells , specific cells that respond to its presence. These

cells possess the receptors needed to bind and "read" the hormonal message. The

other hormones are treated like junk mail and ignored, because the cell lacks the

receptors to read the messages they contain. The use of hormones to coordinatecellular activities in tissues in distant portions of the body is called endocrine

communication .

A hormone may

stimulate the synthesis of an enzyme or a structural protein not already

present in the cytoplasm by activating appropriate genes in the cell nucleus;

increase or decrease the rate of synthesis of a particular enzyme or other

protein by changing the rate of transcription or translation; or

turn an existing enzyme "on" or "off" by changing its shape or structure.

The nervous system also relies primarily on chemical communication, but it does

not use the bloodstream to deliver messages. Instead, neurons release a

neurotransmitter at a synapse very close to the target cells that bear the

appropriate receptors. This form ofsynaptic communication is ideal for crisis

management: If you are in danger of being hit by a speeding bus, the nervous

system can coordinate and direct your leap to safety. The differences between the

nervous and endocrine systems seem relatively clear.

Both systems rely on the release of chemicals that bind to specific receptors

on their target cells.

The two systems share many chemical messengers; for example,

norepinephrine and epinephrine are called hormones when released into the

bloodstream, but neurotransmitters when released across synapses.

Both systems are regulated primarily by negative feedback control

mechanisms.

The two systems share a common goal: to preserve homeostasis by

coordinating and regulating the activities of other cells, tissues, organs, and

systems
http://www.emedicinehealth.com/script/main/art.asp?articlekey=4359http://www.emedicinehealth.com/script/main/art.asp?articlekey=4359


6/17

The major glands of the endocrine system are the hypothalamus, pituitary, thyroid,

parathyroids, adrenals, pineal body, and the reproductive organs (ovaries and

testes). The pancreas is also a part of this system; it has a role in hormone

production as well as in digestion.

The endocrine system is regulated by feedback in much the same way that a

thermostat regulates the temperature in a room. For the hormones that are

regulated by the pituitary gland, a signal is sent from the hypothalamus to the

pituitary gland in the form of a "releasing hormone," which stimulates the pituitary

to secrete a "stimulating hormone" into the circulation. The stimulating hormone

then signals the target gland to secrete its hormone. As the level of this hormone

rises in the circulation, the hypothalamus and the pituitary gland shut down

secretion of the releasing hormone and the stimulating hormone, which in turn

slows the secretion by the target gland. This system results in stable blood

concentrations of the hormones that are regulated by the pituitary gland.

Hypothalamus

The hypothalamus is located in the lower central part of the brain. This part of the

brain is important in regulation of satiety, metabolism, and body temperature. In

addition, it secretes hormones that stimulate or suppress the release of hormones

in the pituitary gland. Many of these hormones are releasing hormones, which are

secreted into an artery(the hypophyseal portal system) that carries them directly to

the pituitary gland. In the pituitary gland, these releasing hormones signal secretion

of stimulating hormones. The hypothalamus also secretes a hormone

calledsomatostatin, which causes the pituitary gland to stop the release of growth

hormone.

Pituitary Gland

The pituitary gland is located at the base of the brain beneath the hypothalamus

and is no larger than a pea. It is often considered the most important part of the

endocrine system because it produces hormones that control many functions of

other endocrine glands. When the pituitary gland does not produce one or more of

its hormones or not enough of them, it is called hypopituitarism.

The pituitary gland is divided into two parts: the anterior lobe and

the posteriorlobe. The anterior lobe produces the hormones, which are regulated

by the hypothalamus:

Thyroid-stimulating hormone (TSH) - Stimulates the thyroid gland to

produce thyroid hormones. TSH is released in response to thyrotropin

releasing hormone (TRH) from the hypothalamus. As circulating

concentrations of thyroid hormones rise, the rates

of TRH and TSH production decline. (A lack of thyroid hormones either
http://www.emedicinehealth.com/script/main/art.asp?articlekey=21320http://www.emedicinehealth.com/script/main/art.asp?articlekey=102119http://www.emedicinehealth.com/script/main/art.asp?articlekey=8974http://www.emedicinehealth.com/script/main/art.asp?articlekey=3783http://www.emedicinehealth.com/script/main/art.asp?articlekey=3403http://www.emedicinehealth.com/script/main/art.asp?articlekey=12333http://www.emedicinehealth.com/script/main/art.asp?articlekey=3594http://www.emedicinehealth.com/script/main/art.asp?articlekey=2735http://www.emedicinehealth.com/script/main/art.asp?articlekey=2516http://www.emedicinehealth.com/script/main/art.asp?articlekey=2339http://www.emedicinehealth.com/script/main/art.asp?articlekey=20762http://www.emedicinehealth.com/script/main/art.asp?articlekey=9248http://www.emedicinehealth.com/script/main/art.asp?articlekey=9277http://www.emedicinehealth.com/script/main/art.asp?articlekey=21320http://www.emedicinehealth.com/script/main/art.asp?articlekey=102119http://www.emedicinehealth.com/script/main/art.asp?articlekey=8974http://www.emedicinehealth.com/script/main/art.asp?articlekey=3783http://www.emedicinehealth.com/script/main/art.asp?articlekey=3403http://www.emedicinehealth.com/script/main/art.asp?articlekey=12333http://www.emedicinehealth.com/script/main/art.asp?articlekey=3594http://www.emedicinehealth.com/script/main/art.asp?articlekey=2735http://www.emedicinehealth.com/script/main/art.asp?articlekey=2516http://www.emedicinehealth.com/script/main/art.asp?articlekey=2339http://www.emedicinehealth.com/script/main/art.asp?articlekey=20762http://www.emedicinehealth.com/script/main/art.asp?articlekey=9248http://www.emedicinehealth.com/script/main/art.asp?articlekey=9277


7/17

because of a defect in the pituitary or the thyroid itself is

called hypothyroidism.)

Thyroid Gland

The thyroid gland is located in the lower front part of the neck below the Adam's

apple. It produces thyroid hormones that regulate the body's metabolism. Thesehormones then travel through the bloodstream to all the other tissues and organs to

help control metabolism in adults and growth, metabolism and development of

the brain and nervous systemin children. The pituitary gland controls the release of

thyroid hormones. Thyroid hormones also help maintain normal blood

pressure, heart rate, digestion, muscle tone, and reproductive functions.

The thyroid is shaped like a butterfly. The two "wings" of the butterfly are

the right and left lobes of the thyroid, with lie on both sides of the trachea or

main breathing tube. The connection between the wings is called the isthmus.

The two hormones that the thyroid produces are L-thyroxine(T4) and tri-

iodothyronine (T3).

The thyroxine (T4) and tri-iodothyronine (T3) hormones regulate your body's

metabolic functions such as heat generation, and the utilization ofcarbohydrates,

proteins, and fats. In children, thyroid hormones are responsible for growth and

development.

Regulatory hormones from different parts of the brain control the thyroid's

production of T4 and T3. In the pituitary gland, thyrotropin-stimulating hormone

(TSH) is released when more thyroid hormone is needed and travels via the

bloodstream to the thyroid gland. TSH then stimulates the thyroid to produce T4

and T3. The pituitary gland acts like a thermostat. When there is too much thyroid

hormone in the bloodstream, the pituitary releases less TSH to signal the thyroid

to produce less thyroid hormone. When there is too little thyroid hormone in the

bloodstream, the pituitary releases more TSH to signal the thyroid to increase

thyroid hormone production. Through this "feedback" system, the production of

thyroid hormone is tightly controlled.
http://www.emedicinehealth.com/script/main/art.asp?articlekey=80651http://www.emedicinehealth.com/script/main/art.asp?articlekey=16929http://www.emedicinehealth.com/script/main/art.asp?articlekey=60153http://www.emedicinehealth.com/script/main/art.asp?articlekey=2486http://www.emedicinehealth.com/script/main/art.asp?articlekey=2486http://www.emedicinehealth.com/script/main/art.asp?articlekey=3674http://www.emedicinehealth.com/script/main/art.asp?articlekey=5791http://www.emedicinehealth.com/script/main/art.asp?articlekey=15381http://www.emedicinehealth.com/script/main/art.asp?articlekey=80651http://www.emedicinehealth.com/script/main/art.asp?articlekey=16929http://www.emedicinehealth.com/script/main/art.asp?articlekey=60153http://www.emedicinehealth.com/script/main/art.asp?articlekey=2486http://www.emedicinehealth.com/script/main/art.asp?articlekey=2486http://www.emedicinehealth.com/script/main/art.asp?articlekey=3674http://www.emedicinehealth.com/script/main/art.asp?articlekey=5791http://www.emedicinehealth.com/script/main/art.asp?articlekey=15381


8/17

Location and picture of the thyroid gland. Note two lobes of the thyroid, similar to

butterfly wings.

The Thyroid Gland curves across the anterior surface of the trachea just inferior to

the thyroid cartilage , which forms most of the anterior surface of the larynx


9/17

The two lobes of the thyroid gland are united by a slender connection,

the isthmus.

Thyroid Follicles and Thyroid Hormones

The thyroid gland contains large numbers ofthyroid follicles, spheres lined by asimple cuboidal epithelium. Follicle cells synthesize a globular protein

called thyroglobulin and secrete it into the colloid of the thyroid follicles. Each

thyroglobulin molecule contains the amino acid tyrosine, the building block of

thyroid hormones.


10/17

The formation of thyroid hormones involves three basic steps:

1. Iodide ions are absorbed from the diet at the digestive tract and are delivered

to the thyroid gland by the bloodstream. Carrier proteins in the basal

membrane of the follicle cells transport iodide ions into the cytoplasm.

Normally, the follicle cells maintain intracellular concentrations of iodide that

are many times higher than those in the extracellular fluid.

2. The iodide ions diffuse to the apical surface of each follicle cell, where they

are converted to an activated form of iodide by the enzyme thyroidperoxidase . This reaction sequence also attaches one or two iodide ions to

the tyrosine molecules of thyroglobulin.

3. Tyrosine molecules to which iodide ions have been attached are paired,

forming molecules of thyroid hormones that remain incorporated into

thyroglobulin. The pairing process is probably performed by thyroid

peroxidase. The hormone thyroxine, also known as tetraiodothyronine , or


11/17

contains four iodide ions. Triiodothyronine , or is a related molecule

containing three iodide ions. Eventually, each molecule of thyroglobulin

contains four to eight molecules of or hormones or both.

The major factor controlling the rate of thyroid hormone release is the concentrationof TSH in the circulating blood . TSH stimulates iodide transport into the follicle cells

and stimulates the production of thyroglobulin and thyroid peroxidase. TSH also

stimulates the release of thyroid hormones. Under the influence of TSH, the

following steps occur:

1. Follicle cells remove thyroglobulin from the follicles by endocytosis.

2. Lysosomal enzymes break the thyroglobulin down, and the amino acids and

thyroid hormones enter the cytoplasm. The amino acids are then recycled

and used to synthesize thyroglobulin.

3. The released molecules ofT3 andT4 diffuse across the basement membrane

and enter the bloodstream. About 90 percent of all thyroid secretions is is

secreted in comparatively small amounts.

4. Roughly 75 percent of theT4 molecules and 70 percent of theT3 molecules

entering the bloodstream become attached to transport proteins

called thyroidbinding globulins ( TBGs ). Most of the rest of

theT4andT3 in the circulation is attached to transthyretin , also known

as thyroidbinding prealbumin ( TBPA ), or to albumin , one of the plasma

proteins. Only the relatively small quantities of thyroid hormones that remain

unboundroughly 0.3 percent of the circulatingT3 and 0.03 percent of the

circulatingT4 are free to diffuse into peripheral tissues.

Functions of Thyroid HormonesThyroid hormones readily cross cell membranes, and they affect almost every cell in

the body. Inside a cell, they bind to (1) receptors in the cytoplasm, (2) receptors on

the surfaces of mitochondria, and (3) receptors in the nucleus.

Thyroid hormones bound to cytoplasmic receptors are essentially held in

storage. If intracellular levels of thyroid hormones decline, the bound thyroid

hormones are released into the cytoplasm.

The thyroid hormones binding to mitochondria increase the rates of

mitochondrial ATP production.

The binding to receptors in the nucleus activates genes that control the

synthesis of enzymes involved with energy transformation and utilization.One specific effect of binding to nuclear receptors is the accelerated

production of sodium potassium ATPase, the membrane protein responsible

for the ejection of intracellular sodium and the recovery of extracellular

potassium. As we noted in Chapter 3 , this exchange pump consumes large

amounts of ATP.

Thyroid hormones also activate genes that code for the synthesis of enzymes

involved in glycolysis and ATP production. This effect, coupled with the direct effect


12/17

of thyroid hormones on mitochondria, increases the metabolic rate of the cell.

Because the cell consumes more energy and because energy use is measured

in calories , the effect is called the calorigenic effect of thyroid hormones. When

the metabolic rate increases, more heat is generated. In young

children, TSH production increases in cold weather; the calorigenic effect may help

them adapt to cold climates. (This response does not occur in adults.) In growingchildren, thyroid hormones are also essential to normal development of the skeletal,

muscular, and nervous systems.

T3 Versus T4

The thyroid gland produces large amounts of T4 but T3 is primarily responsible for

the observed effects of thyroid hormones: a strong, immediate, and shortlived

increase in the rate of cellular metabolism. Peripheral tissues have two sources

of T3

1. Release by the Thyroid Gland. At any moment,T3 from the thyroid gland

accounts for only 1015 percent of the in peripheral tissues.

2. The Conversion of to Enzymes in the liver, kidneys, and other tissues can

convertT4 toT3. Roughly 8590 percent of theT3 that reaches the target

cells is produced by the conversion ofT4 within peripheral tissues.

Iodine and Thyroid Hormones

Iodine in the diet is absorbed at the digestive tract. The thyroid follicles contain

most of the iodide reserves in the body. The typical diet in the United

States provides approximately 500ug of iodide per day, roughly three times the


13/17

minimum daily requirement. Much of the excess is due to the addition of to the

table salt sold in grocery stores as "iodized salt." Thus, iodide deficiency is seldom

responsible for limiting the rate of thyroid hormone production. (This is not

necessarily the case in other countries.) Excess is removed from the blood at the

kidneys, and each day a small amount of (about 20ug ) is excreted by the liver into

the bile, an exocrine product stored in the gallbladder. Iodide excreted at thekidneys is eliminated in urine; the iodine excreted in bile is eliminated with

intestinal wastes.

The C Cells of the Thyroid Gland: Calcitonin

A second population of endocrine cells lies sandwiched between the cuboidal follicle

cells and their basement membrane. These cells, which are larger than those of the

follicular epithelium and do not stain as clearly, are theC ( clear ) cells ,

orparafollicular cells. C cells produce the hormone calcitonin ( CT ), which aids in

the regulation of concentrations in body fluids. We introduced the functions of

this hormone in Chapter 6 . The net effect of calcitonin release is a drop in the

concentration in body fluids, accomplished by (1) the inhibition of osteoclasts, which

slows the rate of release from bone, and (2) the stimulation and then falloff of

excretion at the kidneys.

Calcitonin is probably most important during childhood, when it stimulates bone

growth and mineral deposition in the skeleton. It also appears to be important in

reducing the loss of bone mass (1) during prolonged starvation and (2) in the late

stages of pregnancy, when the maternal skeleton competes with the developing

fetus for calcium ions absorbed by the digestive tract.

Thyroid Gland Disorders

Normal production of thyroid hormones establishes the background rates of cellular

metabolism. These hormones exert their primary effects on metabolically active

tissues and organs, including skeletal muscles, the liver, and the kidneys. The

inadequate production of thyroid hormones is called hypothyroidism . In an

infant, hypothyroidism produces cretinism , a condition marked by inadequate

skeletal and nervous development and a metabolic rate as much as 40 percent

below normal levels. The condition affects approximately 1 birth out of every 5000.

Cretinism developing later in childhood will retard growth and mental development

and delay puberty. Adults with hypothyroidism are lethargic and unable to toleratecold temperatures. The symptoms of adult hypothyroidism, collectively known

as myxedema, include subcutaneous swelling, dry skin, hair loss, low body

temperature, muscular weakness, and slowed reflexes. Hypothyroidism may also be

associated with the enlargement of the thyroid gland, producing a distinctive

swelling called a goiter. Hypothyroidism, myxedema, and goiter as the result of

inadequate dietary iodide are very rare in the United States, in part due to the


14/17

addition of iodine to table salt, but these conditions can be relatively common in

poorer countries, especially landlocked ones (seafood is a good source of iodine).

Thyroid Disorders.

(a) Cretinism, or congenital hypothyroidism, results from thyroid hormone

insufficiency in infancy. (b) An enlarged thyroid gland, or goiter, can be associated

with thyroid hyposecretion due to iodine insufficiency in adults.

Hyperthyroidism , or thyrotoxicosis , occurs when thyroid hormones are produced

in excessive quantities. The metabolic rate climbs, and the skin becomes flushed

and moist with perspiration. Blood pressure and heart rate increase, and the

heartbeat may become irregular as circulatory demands escalate. The effects on

the central nervous system make the individual restless, excitable, and subject to

shifts in mood and emotional states. Despite the drive for increased activity, the

person has limited energy reserves and fatigues easily. Graves' disease is a form ofhyperthyroidism that afflicted President George W. H. Bush and Barbara Bush

during their stay in the White House.

Thyroid gland


15/17

The Thyroid gland is one of the largest endocrine glands in the body. It is

positioned on the neck just below the Larynx and has two lobes with one on either

side of the trachea. It is involved in the production of the hormones T3

(triiodothyronine) and T4 (thyroxine). These hormones increase the metabolic

activity of the bodys cells. The thyroid also produces and releases the hormone

calcitonin (thyrocalcitonin) which contributes to the regulation of blood calciumlevels. Thyrocalcitonin or calcitonin decreases the concentration of calcium in the

blood. Most of the calcium removed from the blood is stored in the bones.

The thyroid hormone consists of two components, thyroxine and iodine. This

hormone increases the metabolism of most body cells. A deficiency of iodine in the

diet leads to the enlargement of the thyroid gland, known as a simple goiter.

Hypothyroidism during early development leads to cretinism. In adults, it produces

myxedema, characterized by obesity and lethargy. Hyperthyroidism leads to a

condition known as exophthalmic goiter, characterized by weight loss as well as

hyperactive and irritable behavior.

The thyroid gland is a two-lobed gland that manifests a remarkably powerful activetransport mechanism for up-taking iodide ions from the blood. As blood flows

through the gland, iodide is converted to an active form of iodine. This iodine

combines with an amino acid called tyrosine. Two molecules of iodinated tyrosine

then combine to form thryroxine. Following its formation, the thyroxine becomes

bound to a polysaccharide-protein material called thyroglobulin. The normal thyroid

gland may store several weeks supply of thyroxine in this bound form. An enzymatic

splitting of the thyroxine from the thyroglobulin occurs when a specific hormone is

released into the blood. This hormone, produced by the pituitary gland, is known as

thyroid-stimulating hormone (TSH). TSH stimulates certain major rate-limiting steps

in thyroxine secretion, and thereby alters its rate of release. A variety of bodilydefects, either dietary, hereditary, or disease induced, may decrease the amount of

thyroxine released into the blood. The most popular of these defects is one that

results from dietary iodine deficiency. The thyroid gland enlarges, in the continued

presence of TSH from the pituitary, to form a goiter. This is a futile attempt to

synthesize thyroid hormones, for iodine levels that are too low. Normally, thyroid

hormones act via a negative feedback loop on the pituitary to decrease stimulation

of the thyroid. In goiter, the feedback loop cannot be in operation - hence continual

stimulation of the thyroid and the inevitable protuberance on the neck. Formerly,

the principal source of iodine came from seafood. As a result, goiter was prevalent

amongst inland areas far removed from the sea. Today, the incidence of goiter has

been drastically reduced by adding iodine to table salt.

Thyroxine serves to stimulate oxidative metabolism in cells; it increases the oxygen

consumption and heat production of most body tissues, a notable exception being

the brain. Thyroxine is also necessary for normal growth. The most likely

explanation being that thyroxine promotes the effects of growth hormone on

protein synthesis. The absence of thyroxine significantly reduces the ability of

growth hormone to stimulate amino acid uptake and RNA synthesis. Thyroxine also


16/17

plays a crucial role in the closely related area of organ development, particularly

that of the central nervous system.

If there is an insufficient amount of thyroxine, a condition referred to as

hypothyroidism results. Symptoms of hypothyroidism stem from the fact that there

is a reduction in the rate of oxidative energy-releasing reactions within the body

cells. Usually the patient shows puffy skin, sluggishness, and lowered vitality. Other

symptoms of hypothyroidism include weight gain, decreased libido, inability to

tolerate cold, muscle pain and spasm, insomnia and brittle nails. Hypothyroidism in

children, a condition known as cretinism, can result in mental retardation, dwarfism,

and permanent sexual immaturity. Sometimes the thyroid gland produces too much

thyroxine, a condition known as hyperthyroidism. This condition produces

symptoms such as an abnormally high body temperature, profuse sweating, high

blood pressure, loss of weight, irritability, and muscular pain and weakness. It also

causes the characteristic symptom of the eyeballs protruding from the skull called

exopthalmia. This is surprising because it is not a symptom usually related to a fast

metabolism. Hyperthyroidism has been treated by partial removal or by partialradiation destruction of the gland. More recently, several drugs that inhibit thyroid

activity have been discovered, and their use is replacing the former surgical

procedures. Unfortunately thyroid conditions require lifetime treatment and

because of the body's need for a sensitive balance of thyroid hormone both

supplementing and suppressing thyroid function can take months or even years to

regulate.

T3 and T4 Function within the body

The Production of T3 and T4 are regulated by thyroid stimulating hormone (TSH),

released by the pituitary gland. TSH Production is increased when T3 and T4 levelsare too low. The thyroid hormones are released throughout the body to direct the

body's metabolism. They stimulate all cells within the body to work at a better

metabolic rate. Without these hormones the body's cells would not be able to

regulate the speed at which they performed chemical actions. Their release will be

increased under certain situations such as cold temperatures when a higher

metabolism is needed to generate heat. When children are born with thyroid

hormone deficiency they have problems with physical growth and developmental

problems. Brain development can also be severely impaired

The significance of iodine

Thyroid hormone cannot be produced without an abundant source of iodine. The

iodine concentration within the body, although significant, can be as little as 1/25th

the concentration within the thyroid itself. When the thyroid is low on iodine the

body will try harder to produce T3 and T4 which will often result in a swelling of the

thyroid gland, resulting in a goiter.


17/17

Secretion and Distribution of Hormones

Hormone release typically occurs where capillaries are abundant, and thehormones quickly enter the bloodstream for distribution throughout the body. A

freely circulating hormone remains functional for less than one hour, and

sometimes for as little as two minutes. It is inactivated when (1) it diffuses out of

the bloodstream and binds to receptors in target tissues, (2) it is absorbed and

broken down by cells of the liver or kidneys, or (3) it is broken down by enzymes in

the plasma or interstitial fluids.

Thyroid hormones and steroid hormones remain in circulation much longer,

because when these hormones enter the bloodstream, almost all of them become

attached to special transport proteins. Thus, the bloodstream contains a substantial

reserve (several weeks' supply) of these hormones at any time.

Endocrine System Introduction

Documents

Transcript of Endocrine System Introduction