MODULE 13- The Endocrine System
The endocrine system is responsible for the release and regulation of the hormones that
circulate throughout your body. As you may have noticed, it is essential to control the
levels of hormones circulating to maintain normal health and homeostasis. Normally, in a
healthy person, these levels do not fluctuate too much but are tightly regulated via
negative feedback, with a few exceptions.
When hormone levels are altered, physiological consequences are inevitable
(such as too much thyroid hormone or too much cortisol). Remember that the nervous
system and endocrine system act together to “signal” and maintain homeostasis…with
the nervous system being very quick, producing almost immediate results, while the
endocrine system is much slower, but its effects can last much longer. The reason the
endocrine system is slower is because it is like driving the country roads to a destination
through the winding, scenic routes of the bloodstream rather than the direct 401, no
fooling around pathway down a neuron. But, since the hormones circulate around and
around and around until they are either eliminated by the kidney or destroyed by
enzymes in the body, they can have longer lasting effects.
1) What is a Hormone?
A hormone is a substance that makes its journey through the BLOODSTREAM
and produces a “signal” at a distant site. It is released by endocrine tissue such as the
kidney, pancreas, pituitary, gonads, hypothalamus, thyroid, and adrenal glands. Several
classes of hormones exist, which differ in their location of the receptor and properties of
2) If hormones travel through the bloodstream, why don’t they affect every single
organ in the body? For example, why does thyroid stimulating hormone affect the
thyroid and not the adrenal glands?
In order to “respond” to a hormone, you must have a receptor that can bind the
hormone. No receptor means no action! Since the thyroid has receptors that bind thyroid
stimulating hormone (TSH) it will produce a response to TSH (and secrete T3 and T4).
However, the adrenal gland does not have receptors for TSH and therefore, nothing
happens when TSH is present in the bloodstream of the adrenal glands.
3) Classes of Hormones
These are subdivided mainly based on their SOLUBILITY. If you remember back
in Modules 2 and 3, lipophilic (or hydrophobic) substances can dissolve right through cell
membranes while hydrophilic substances do not. So, you have lipophilic (lipid-
loving/hydrophobic) and hydrophilic hormones. Just based on that tidbit of information, it
is probably easy for you to determine each of their properties.
A) If a hormone is hydrophilic then...they dissolve in the bloodstream easily
(because they like the watery environment) and can’t cross the cell membrane.
So where would your receptor have to be? ON THE OUTSIDE OF THE CELL.
Examples of this class of hormones are all protein hormones, including insulin,
glucagons (pancreatic hormones), pituitary hormones (GH, prolactin, TSH, LH,
FSH etc), parathyroid hormone, and most of the hypothalamic hormones (for this
course, consider all the hypothalamic hormones in this category). These
hormones are derived from amino acids. B) If a hormone is lipophilic/hydrophobic then…they easily dissolve into cells
but do not like to float in the ciruculation because it is a watery environment.
Because of this, these hormones need to circulate bound to a protein carrier
(these protein are hydrophilic and help the lipophillic hormones dissolve in the
blood). Receptors for these hormones are located INSIDE THE CELL (they don’t
need a receptor on the cell membrane surface to carry them into the cell). So the
lipophillic hormones diffuse right inside and either binds to receptors in the cell’s
cytoplasm or on the nucleus of the cell.
Examples of these types of hormones are steroid hormones including those of
the adrenal cortex, as well as sex steroids testosterone, estrogen, progesterone.
These hormones are all derived from cholesterol. Interestingly enough, thyroid
hormones (T3 and T4) also fall into this category even though they are made up
of the amino acid tyrosine. Rather than behaving like a protein hormone, these
4) How Do Hormone Receptors Work? How Can They Tell the Cells What To Do?
This is a bit complicated. Obviously if you are a receptor on the outside of a cell, it is
harder to get the message into the cell than if you are already inside. If you are “on the
outside” you must use a “messenger” to get the message to the inside of the cell. Most
receptors have a “messenger” built into them, such as G proteins that can produce
cAMP or open ion channels, or tyrosine kinases. When the hormone binds to its receptor
outside the cell, the “messenger” causes a change in the activity of the cell and ALTERS
proteins that are already present within the cell.
But if you are “on the inside”, there is no need for a messenger. Once the hormone
diffuses into the cell and binds to its receptor, this complex of hormone+receptor can
bind to DNA (the genetic material in a cell) and change the cell’s activity and increase or
decrease the production of new proteins.
Don’t worry, I won’t ask the exact mechanisms of how cAMP or tyrosine kinase works
but know what I’ve mentioned above.
5) The Main Structures in Hormone Regulation and Secretion, the Hypothalamus
You’ve probably noticed that the hypothalamus plays a big part in almost every
system, from temperature control to osmolarity regulation, to hunger and thirst, to total
body volume regulation and now hormone regulation. The hypothalamus is often been
referred to as the “little brain” which surprisingly is less than 1% of the weight of the
entire brain itself. In this module, you learn about how important it is in controlling the
anterior and posterior pituitary.
The hypothalamus secretes many different hormones. These include the
releasing factors that control the anterior pituitary. However, the hypothalamus also
synthesizes the hormones oxytocin and ADH (also known as vasopressin) that are
released from the posterior pituitary.
6) Are the Posterior and Anterior Pituitary Different?
The pituitary gland is made up of two different “tissues”. Seems funny for a gland
that is so small, but both regions are very different. A) The Anterior Pituitary-
The anterior pituitary is made up of endocrine tissue and secretes a lot of
different hormones in response to the hypothalamic releasing/inhibiting
hormones. It “receives” the hypothalamic hormones via a special circulatory
system that comes from the hypothalamus and circulates directly to the anterior
pituitary. This special circulation is called the hypothalamic-hypophyseal portal
system. So the hypothalamic releasing/inhibiting hormones hop into the
hypothalamic-hypophyseal portal system and are delivered to the anterior
pituitary. The anterior pituitary then responds by secreting hormones in response
to a particular hypothalamic hormone. The hormone from the anterior pituitary
then hops into the general systemic circulation directly to circulate throughout the
B) The Posterior Pituitary
The posterior pituitary is actually considered to be neural tissue. There are
neurons that have their cell bodies in the hypothalamus BUT their axons are sent
down into the posterior pituitary (since the hypothalamus and pituitary are so
close together). The posterior pituitary releases the hormones oxytocin and ADH
(that are made in the hypothalamus) out of the axon terminals found in the
posterior pituitary into general systemic circulation. So if action potentials occur in
this region, you will have hormones released. You already know the actions of
ADH from the kidney section, but oxytocin is responsible for uterine contractions
during labour and the release of milk during nursing (this hormone is not yet
known to function in males or in non-pregnant females).
7) Anterior Pituitary hormones:
As I said before, the anterior pituitary releases certain hormones depending on what
hypothalamic hormones are “delivered”. Here is the actual list (see 13.21).
If the anterior pituitary receives___(a)___ from the hypothalamus, it secretes
(a) Thyroid releasing hormone (TRH) ----- (b) Thyroid stimulating hormone (TSH)
(a) Corticotropin releasing hormone (CRH) ----- (b) Adrenocorticotropic hormone (ACTH)
(a) Prolactin releasing hormone (PRH) ----- (b) Prolactin
(a) Prolactin inhibiting hormone (PIH) ---- also affects prolactin
(a) Growth hormone releasing hormone (GHRH) ---- (b) growth hormone (GH)
(a) Growth hormone inhibiting hormone (GHIH) ----also affects growth hormone
(a) Gonadotropin releasing hormone (GnRH) ---- (b) Luteinizing hormone (LH) & Follicle
stimulating hormone (FSH)
Note: I don’t want to give you the impression that only 1 hypothalamic hormone is
“received” by the pituitary at one particular time. Rather several hypothalamic hormones
can be delivered continuously or in pulses depending on the requirements by the body
Seems easy, right? However, it is a bit more complicated than that. To maintain
appropriate levels of hormones for homeostasis, there is some feedback of (b) on (a) in
some cases. This means if there is too much of hormone (b) present, it will shut off the
release of hormone (a) through short loop feedback. This generally occurs for the hormones that do not go on to secrete a third hormone (so prolactin and growth