Section 13.1 – Objectives
• Define endocrine gland and hormone and explain the general role of the endocrine
• List the characteristic functions of hormones.
• Identify the three chemical types into which hormones fall.
• Describe how the hormones are secreted, how they exert their effects, and how
they are inactivated.
• Describe the chemical nature of the three types of hormones and the significance
of the chemical structure to how the hormone performs its function.
• Define hormone receptors, identify where they are found, and describe their
• Explain how the three chemical types of hormones each affect their target cells
(that is, their mechanism of action).
• Explain how hormones are metabolized/removed from the circulation.
• Identify the control mechanism responsible for regulating hormone secretion and
explain how it works.
• Describe the location and general structure of the hypothalamus.
• List the functions of the hypothalamus as they pertain to maintaining homeostasis
in the body.
• List the hormones that are secreted by the hypothalamus.
• Draw a picture of the pituitary gland, showing the anterior and posterior sections.
Explain how these sections are related to the hypothalamus.
• Explain why the pituitary is considered the master gland of the body.
• List the hormones that are produced by the anterior and posterior pituitary and
their ultimate sites of action.
• Explain how the hormones of the pituitary are regulated.
• Describe the location of the thyroid and its general structure.
• List the two hormones produced by the thyroid gland and describe how they are
• Describe how the hormones of the thyroid gland are secreted and the regulation of
• List the effects of T3 and T4 on the body.
• Describe the diseases of the thyroid gland.
• Describe the location and general structure of the adrenal glands, including a
description of the different layers that make it up. Explain the origin of each layer
and the systems of the body that control them.
• List the hormones that are secreted by the adrenal gland and the type of hormone
each is considered to be.
• Explain the important function of cortisol, how it is secreted, and its effects on the
1 • Explain what Cushing's Syndrome is and its effects on the body.
• Describe the structure and function of the pancreas.
• Describe the release of insulin, glucagon and somatostatin and their effects on the
• Distinguish between type I and type II diabetes mellitus.
Section 13.2 – Introduction
• In the first module, you learned that homeostasis was the process by which
conditions inside the body are maintained at constant levels.
• This was performed by two special systems in the body using negative feedback
1. The nervous system
2. The endocrine system.
• You should have a good grasp of the nervous system, so it is time to turn our
attention to the endocrine system.
• The endocrine system:
Consists of a series of glands that secrete different chemicals into the
These hormones then travel throughout the body to their target site
to initiate their effect.
Section 13.3 - Endocrine System: Function
• The functions of the endocrine system:
the maintenance of the internal environment (body temperature, body
fluid volume, osmolality, and so on)
2 adaptation to stress
control of growth and metabolism
Control of reproduction.
• In contrast to the nervous system, the actions of the endocrine system are slower
to take effect yet last longer and are generally more widespread throughout the
• While the nervous system has control over organs with which it is in direct
contact, the endocrine system can indirectly affect many organs of the body at a
distance by secreting chemicals into the blood.
Section 13.4 – Glands
• An endocrine gland:
Agroup of specialized cells that synthesize, store, and release a very
special chemical into blood.
This chemical is called a hormone.
o The hormone is released into the bloodstream and
circulates throughout the body to specific target cells that
have receptors for the hormone.
o The hormone will then have its effect on the target cell and
can either stimulate or inhibit the activity of the cell.
Section 13.5 – Glands (cont’d)
• There are a number of glands located throughout the body.
• We have already discussed the hypothalamus and pituitary glands in the nervous
• We will examine them again in more detail here.
3 • The other glands in the body include the thyroid, adrenal, pancreas, and the
gonads (ovaries in the female and testes in the male).
• We will examine the male and female gonads in the reproductive module.
Section 13.6 – Hormones
• All hormones can be divided into three basic categories based on their chemical
1. Hormones derived from the amino acid Tyrosine (thyroxine,
triiodothyronine), which are secreted from the thyroid gland
2. Hormones derived from proteins: calcitonin, parathyroid hormone, the
pituitary and pancreatic hormones, and most of the releasing and
inhibiting hormones from the hypothalamus
3. The steroid hormones cortisol, aldosterone, estrogen, progesterone and
testosterone, which are all derived from cholesterol
Section 13.7 – Hormones: Basic Function
• Hormones are secreted into the blood in "pulses" by a very specific stimulus
(neural or blood-borne) and in amounts that vary with the strength of the stimulus.
• Once secreted by the gland, hormones are present in very small concentrations in
• They exert their specific effects by binding to receptors on or in the target cells
and generally act by regulating pre-existing reactions.
4 Section 13.8 – Hormones: Receptors
Aunique structure in or on a cell that interacts with a chemical (in this
case the hormone) in a particular way
Aspecific hormone will bind to a specific receptor.
As we have seen, the type of hormone will determine where the receptor is
located – either on the membrane, in the cytoplasm, or in the nucleus.
This interaction between receptor and hormone will then trigger a
response in the cell.
Section 13.9 – Hormones: Receptors for Hydrophobic Hormones
• Since hydrophobic (lipid soluble) hormones (steroid and thyroid hormones) can
diffuse through the cell membrane, the receptor will be located in the cytoplasm
or in the nucleus.
• However, the hormone first must be released by its carrier protein before it can
enter the cell.
• Once the hormone is inside the cell, it will bind with its receptor, either in the
cytoplasm or in the nucleus.
• This new hormone/receptor complex will then bind to the DNAwithin the nucleus
to eventually alter various activities of the cell.
• These activities could be to increase or decrease production of proteins.
5 Section 13.10 – Hormones: Receptors for Hydrophilic Hormones
• Hydrophilic (protein) hormones are unable to diffuse through the membrane and
therefore must be able to alter the activity of the cell from the "outside."
• The receptors for protein hormones are located on the cell membrane.
• When the hormone attaches to the receptor, it initiates a sequence of chemical
reactions that will eventually alter the activity of the cell.
• There are three ways in which the receptor can affect the cell:
1. Through a second messenger system
2. Through tyrosine kinase
3. Through G-proteins.
Section 13.11 – Hormones: The Second Messenger
• In this situation, when the hormone binds to its receptor, it causes a G-protein on
the inside of the membrane to produce a "second messenger" (the first messenger
is the hormone).
• The most widely studied second messenger is called cyclic adenosine
This second messenger, released into the cytoplasm, will rapidly alter
proteins already present inside the cell.
These altered proteins will then trigger a sequence of reactions
inside the cell, which will lead to a variety of intracellular
responses - including the release of proteins.
Section 13.12 – Hormones: Tyrosine Kinase
• In this case, the hormone receptor complex activates tyrosine kinase on the inside
surface of the membrane.
• The tyrosine kinase, like the second messenger we saw earlier, then alters existing
proteins that will then alter the activity of the cell.
Section 13.13 – Hormones: Ion Channels
• When the hormone attaches to its receptor, a G-protein is activated that lies within
the cell membrane.
This G-protein can then open adjacent ion channels.
If the ion is calcium (Ca ), it can act as a second messenger to
alter existing proteins once it diffuses into the cell.
• Once hormones have had their effects on their target tissue, they are broken down
by different systems in the body.
• Hormones are removed from the blood by the same mechanisms as any other
6 Metabolic destruction in the blood or by tissues (mainly liver and kidney)
Excretion by the liver into the bile
Excretion by the kidneys into the urine.
Section 13.14 – Hormones: Control of Secretion
• Secretion of most hormones is controlled by negative feedback.
• We have seen how negative feedback is involved with the regulation of the
cardiovascular, respiratory, and urinary systems.
• We will see that negative feedback is involved throughout the endocrine system in
controlling the secretion of almost all hormones.
• It is now time to look at each gland, the hormone(s) that it secretes, its effects on
the body, and how it is regulated.
• We will begin with the hypothalamus and work our way down through the body.
Section 13.15 - The Hypothalamus: Structure and Function
• The hypothalamus:
Located at the base of the brain just above the pituitary gland and below
Because of its central location, it can receive information from all over the
It is composed of many regions made up of groups of nerve cell bodies,
which are called nuclei.
Several of these nuclei control the release of hormones from the pituitary
The hypothalamus is involved with some of the body's homeostatic
mechanisms, including the regulation of:
1. Body temperature
2. water balance
3. Energy production.
It is also involved in regulating the behavioral drives of:
3. Sexual behavior.
7 • In order to perform all of these complex and widespread functions, the
hypothalamus receives large amounts of information from all around the body,
including metabolic, hormonal, temperature, and neural information.
Section 13.16 – The Hypothalamus: Hormones and Releasing Factors
• The hypothalamus secretes many types of hormones - sometimes called releasing
factors - into a special portal system that we will see in a moment.
• These include the following:
Prolactin Releasing Hormone (PRH)
Prolactin Inhibiting Hormone (PIH)
Thyrotropin Releasing Hormone (TRH)
Corticotropin Releasing Hormone (CRH)
Growth Hormone Releasing Hormone (GHRH)
Growth Hormone Inhibiting Hormone (GHIH)
Gonadotropin Releasing Hormone (GnRH)
These chemicals are referred to as releasing or inhibiting
hormones because they cause the release or inhibition of a
hormone from the anterior pituitary gland (which we will see in a
o For example, prolactin releasing hormone (PRH) is
secreted from the hypothalamus to cause the "release" of
the hormone prolactin from the anterior pituitary.
o Prolactin then circulates in the blood to affect the target
Section 13.7 – The Pituitary Gland: Structure
• Pituitary Gland:
Divided into two distinct regions:
8 1. The anterior pituitary (AP):
Develops from tissues that form the roof of the mouth and
is made up of endocrine tissue
The endocrine cells in this area secrete pituitary
hormones directly into the blood.
This part of the pituitary is regulated by the hypothalamus
through a very special circulatory system called the
hypothalamic-hypophyseal portal system.
The hypothalamus communicates with the anterior
pituitary by secreting the releasing or inhibiting hormones
into this portal system.
These hormones travel to the anterior pituitary to either
stimulate or inhibit the release of the pituitary hormones.
2. The posterior pituitary
Develops from neural tissue at the base of the brain.
It contains the axons and nerve terminals of neurons
whose cell bodies lie in the hypothalamus.
This tract of neurons is sometimes referred to as the
These neurons produce neurohormones (antidiuretic
hormone and oxytocin) in the hypothalamus that are
secreted into the blood from the posterior pituitary in
response to action potentials.
Section 13.18 – The Pituitary Gland: Structure (cont’d)
• Info about posterior pituitary is above!
Section 13.19 – The Pituitary Gland: Function
• The hormones of the anterior pituitary control many metabolic functions
throughout the body.
• In addition, two other anterior hormones control growth of the ovaries and testes
and regulate their reproductive functions:
1. Antidiuretic hormone:
From the posterior pituitary
Regulates water reabsorption in the kidney.
From the posterior pituitary
Regulates milk release from the breasts
Causes the contraction of the uterus during labor.
Section 13.20 – The Pituitary Gland:Anterior Pituitary Hormones
• Anterior pituitary hormone release is summarized below and in the table on the
9 1. Thyroid releasing hormone (TRH):
Secreted from the hypothalamus
Causes the release of thyroid stimulating hormone (TSH) from the
o TSH then stimulates the thyroid gland to secrete the two
1. triiodothyronine (T3)
2. thyroxine (T4).
Both are involved in metabolism, growth,
and development of many tissues in the
Corticotropin releasing hormone (CRH):
Stimulates the release of adrenocorticotropic hormone (ACTH)
from the anterior pituitary.
o ACTH then principally stimulates the adrenal glands to
o ACTH also has a minor effect in the secretion of adrenal
androgens and aldosterone.
Growth hormone (GH):
Is under the control of two hypothalamic hormones:
1. Growth hormone releasing hormone (GHRH) and
The primary stimulus for GH release is GHRH.
2. Growth hormone inhibiting hormone (GHIH – also
known as somatostatin).
GHIH has a weak inhibitory effect on the release
Gonadotropin releasing hormone (GnRH):
Stimulates the anterior pituitary to secrete both luteinizing
hormone (LH) and follicle stimulating hormone (FSH).
o Both of these hormones act on the testes in the male and
ovaries in the female.
Released from the anterior pituitary
Is under the control of two hypothalamic hormones:
1. Prolactin releasing hormone (PRH)
2. Prolactin inhibiting hormone (PIH):
However, PIH has the strongest control over
IF have time after studying everything, come back to THIS section and take a
look at the animation!!!
10 Section 13.21 - The Pituitary Gland:Anterior Pituitary Hormones (cont`d)
• The table below summarizes the hypothalamic releasing and inhibiting hormones
as well as the anterior pituitary hormones and some of their target tissues.
Section 13.22 – The Pituitary Gland: Posterior Pituitary Hormones
• The hormones secreted from the posterior pituitary are manufactured by nerve
cells whose cell bodies lie in the hypothalamus.
• The hormones are carried down to the terminal end of the nerve and are released
in response to action potentials – much like neurotransmitters are released.
• The two hormones released by the posterior pituitary:
1. antidiuretic hormone (ADH – also called vasopressin):
ADH causes the reabsorption of water in the kidney
They are produced in cell bodies and carried down to nerve
Action potentials in nerves releaseADH from nerve terminal into
Oxytocin is responsible for the ejection of milk from the breasts
and causes the uterus to contract during birth.
Is produced in the cell body of the nerves located in hypothalamus
Oxytocin is then carried down to axon terminal
Action potential in nerves trigger release of oxy in circulation
11 Section 13.23 – The Pituitary Gland: Regulation by Negative Feedback
• It is very important to remember that several hormones are at work at any one
• The hypothalamus secretes a releasing hormone (H1), which causes the release of
an anterior pituitary hormone (H2) into the blood.
• Hormone H2 can feed back to the hypothalamus to decrease the release of
hormone H1 in a "short loop" negative feedback system.
• The anterior pituitary hormone (H2) will circulate to an endocrine gland to cause
the release of another hormone (H3).
• Hormone H3 can feed back to the hypothalamus and pituitary to decrease the
release of hormones H1 and H2, respectively, by a "long loop" negative feedback
• In some cases, the hormone from the endocrine gland (H3) will affect one or more
These target tissue responses can feed back to the hypothalamus and can
decrease the release of its hormone (H1) in an "ultra long loop" reflex.
Section 13.24 – The Pituitary Gland: Regulation by Negative Feedback (cont`d)
• The animation at right shows hypothalamic control of the anterior pituitary and
the release and effect of the hormones prolactin, thyroid stimulating hormone
(TSH), adrenocorticotropic hormone (ACTH), growth hormone (GH), follicle
stimulating hormone (FSH), and luteinizing hormone (LH).
• Thyroid stimulating hormone (TSH):
Thyrotropin releasing hormone is secreted by the hypothalamus to cause
the release of the thyroid stimulating hormone by the anterior pituitary.
TSH then stimulates the thyroid gland to secrete the hormones thyroxine
Once circulating in the blood, both will feed back to the hypothalamus to
decrease the release of TSH and to the pituitary to decrease the release of
thyroid stimulating hormone.
• Adrenocorticotropic hormone (ACTH):
Corticotropin releasing hormone is secreted by the hypothalamus into the
portal system where it will flow to the anterior pituitary to cause the
release of ACTH.
ACTH will then stimulate the adrenal gland to secrete mainly cortisol, but
also, aldosterone and androgens in small amounts.
Cortisol will then feed back to both the hypothalamus and the anterior
pituitary to decrease the release of corticotropin releasing hormone and
• Growth hormone (GH):
GHRH stimulate the release of GH from the anterior pituitary while
growth hormone inhibiting hormone decreases growth hormone release.
12 GH will then feed back to the hypothalamus to decrease the release of
This in turn will decrease the output of growth hormone from the pituitary.
• Follicle stimulating hormone (FSH) and luteinizing hormone (LH):
Gonatotropin releasing hormone (GRH) stimulates the release of both
luteinizing hormone and follicle stimulating hormone from the anterior
LH and FSH travel in the blood to the gonads: the testes in the male and
the ovaries in the female – to stimulate the release of their respective sex
These sex hormones then feed back to the hypothalamus and the pituitary
to inhibit the release of GRH and LH, FSH respectively.
Recall that prolactin is released from the anterior pituitary mainly when
the levels of prolactin inhibiting protein decrease.
Suckling causes a dramatic decrease in levels of prolactin inhibiting
protein which will in turn cause the secretion of prolactin.
Prolactin will circulate to the breasts to maintain the production of milk
and cause its secretion.
When suckling stops the levels of prolactin inhibiting protein increase
causing prolactin levels to decrease.
Section 13.25 – Pituitary Gland:AQuick Review
• It is very important to remember that the hypothalamus controls both the anterior
and posterior pituitary.
• Releasing (or inhibiting) factors are secreted into the special portal system from
the hypothalamus to control the release of hormones from the anterior pituitary.
• Special nerve cells, whose cell bodies are located in the hypothalamus, produce
and store hormones.
• These hormones, which are located in the posterior pituitary, are then released
from the cell's axon terminal into the circulation.
Section 13.26 - Pituitary Gland:AQuick Review
The following are releasing factors/hormones secreted by the hypothalamus:
• Prolactin Releasing Hormone (PRH)
• Prolactin Inhibiting Hormone (PIH)
• Thyrotropin Releasing Hormone (TRH)
• Corticotropin Releasing Hormone (CRH)
• Growth Hormone Releasing Hormone (GHRH)
• Growth Hormone Inhibiting Hormone (GHIH)
• Gonadotropin Releasing Hormone (GnRH)
13 Hormones secreted by the anterior pituitary and released by the posterior pituitary
are listed below.
Section 13.27 – The Thyroid Gland: Structure
• The Thyroid Gland:
Lies directly below the larynx (or voice box) and consists of two lobes that
almost completely surround the trachea.
The thyroid is made up of follicles, which are the functional units of the
These follicles consist of a central region of colloid surrounded by
• Lying between the follicles are parafollicular cells or C
Section 13.28 - The Thyroid Gland: Function
• The thyroid was one of the first organs in the body to be identified as a gland.
• Its primary function, although not an absolute necessity for life, is to produce the
thyroid hormones triiodothyronine (T3) and thyroxine (also called
tetraiodothyronine, or T4).
As their names imply, these hormones contain iodine.
These hormones are responsible for regulating the basal metabolic rate of
You should recall that these hormones are made from the amino acid
tyrosine; this makes them hydrophobic.
Therefore, they require a protein carrier to circulate in the blood,
but they can diffuse through the cell membrane.
14 • Also, the follicular cells of the thyroid gland produce the protein hormone
calcitonin which causes a decrease in calcium levels in the blood.
Section 13.29 – The Thyroid Gland: Production of T3 and T4
• Both T3 and T4 are produced inside the follicles of the thyroid gland by
combining iodine and tyrosine with the help of a glycoprotein called
• The epithelial cells take up molecules of tyrosine from the circulation; here they
combine with thyroglobulin, which is produced within the cells.
• The epithelial cells also actively take up iodine, which is absorbed from the diet.
• As the tyrosine-thyroglobulin complex is secreted into the colloid, one or two
molecules of iodine attach to each tyrosine.
• In the colloid, two tyrosine molecules will join together while attached to the
• The number of iodine molecules attached to the tyrosine will produce either
triiodothyronine (also called T3, with three iodine molecules) or
tetraiodothyronine (also called thyroxine or T4, with four iodine molecules).
Section 13.30 – The Thyroid Gland: Secretion of T3 and T4
15 • Thyroid stimulating hormone (TSH), released from the anterior pituitary gland,
will bind to a receptor on the membrane of the epithelial cell which will stimulate
a number of reactions.
• These reactions include:
The trapping and taking up of circulating iodine in order to form the
hormones (not shown at right)
Stimulating endocytosis of the T3- and T4-thyroglobulin complex into the
The enzymatic removal of the thyroglobulin from T3 and T4 in the
epithelial cells and stimulating t