Hormones are secreted (usually
into the bloodstream) by a collection of glands
inside the body referred to as the endocrine
system. (A gland is a group of cells that
produces and secretes chemicals into the body.)
The major glands that make up the endocrine
system include the hypothalamus, the pituitary
gland, the thyroid and parathyroids, the
adrenals, the pineal body, and the ovaries and
testes (the gonads).
Hormones can also be produced synthetically
in a laboratory setting and are prescribed by
doctors to treat disease or hormone
deficiencies. For example, if a person has had
their thyroid gland removed, a doctor may
prescribe synthetic thyroid hormones to replace
those that the person's body can no longer
produce.
Over fifty different hormones have been
identified in the bodies of humans, and more are
still being discovered. Hormones influence and
regulate practically every cell, tissue, organ,
and function of our bodies, including growth,
development, metabolism, maintenance/balance of
our internal environment (homeostasis), and
sexual and reproductive function.
How do hormones cause their
effects?
Most hormones circulate via the blood, thus
coming into contact with all kinds of cells
throughout the body. However, a given hormone
usually affects only a limited number of cells,
which are called "target cells" for that
hormone. A target cell responds to a hormone
because it bears "receptors" for that hormone.
Hormones, like all molecules, have a specific
molecular shape, and thus will fit into certain
receptors but not others (see illustration
right).
When it binds to the receptor site of a
target cell, a molecule might act as an
"agonist" or as an "antagonist" (or some
combination of the two).
"Agonists" are molecules that bind to the
receptor site of a target cell and produce
biological effects as a result. For example,
when the hormone testosterone is secreted into
the blood and binds to a target cell's receptor
site, biological effects from that binding will
result in producing a specific physical change
(such as the stimulation of a hair follicle to
produce a whisker on the chin). Testosterone is
an example of an agonist in this case.
Bear in mind that in many cases, more than
one distinct hormone can bind to the same
receptor. For a given receptor, different
agonists can have dramatically different
potencies.
"Antagonists" are molecules that bind to the
receptor site of a target cell while at the same
time failing to trigger the biochemical results
of the agonist.
Antagonist molecules may compete with an
agonist for receptor sites, thus preventing or
blocking the binding of an agonist. For this
reason, antagonist compounds are often used as
drugs.
An example of an antagonist is the drug
Tamoxifen, which serves as an estrogen-receptor
antagonist in breast tissue (it is sometimes
called an "anti-estrogen" in medical
literature). Tamoxifen is used in the treatment
of breast cancer. It binds to estrogen target
cell receptors in the breasts and blocks the
ability of estrogen to produce its biologic
effects-one of which is the feeding of the
cancer itself.
Interestingly enough, while Tamoxifen acts as
an estrogen antagonist in breast tissue, it also
acts as an estrogen agonist in the bones. That
is, Tamoxifen produces a mixture of
antagonist/agonist reactions, blocking the
actions of estrogen in the breasts, while also
producing the (positive) biologic effects of
estrogen in the bones.
How are hormones regulated in the
body?
The production of hormones in the body is
almost always regulated by a delicate set of
feedback relationships, or "feedback loops."
Most (but not all) hormone secretion is governed
by "negative" feedback loops, wherein the amount
of a substance in a system regulates its own
concentration. When concentration of a hormone
rises to above desired levels, a series of steps
are taken within the system to cause the
concentration to fall. Conversely, steps are
taken to increase concentration when the level
is too low.
Imagine a home-heating system as an example
of a simple negative feedback loop. When the
temperature of a room rises above the set point
of a thermostat, the thermostat is then
triggered and shuts off the furnace (the heat
feeds back negatively on the source of heat).
When temperature drops back below the set point,
negative feedback is gone, and the furnace turns
back on to produce more heat.
The above example of a feedback system is
quite simplified; the body relies on complex
positive and negative feedback systems, often
involving multiple different hormones, steps,
and tissues to regulate bodily functions.
In order to function, the body needs healthy
endocrine glands that work correctly, a properly
functioning blood supply to move hormones
through the body to their target points,
receptor sites on the target cells for the
hormones to do their work, and a feedback system
for controlling how and when hormones are
produced and used. Any disruption in that system
can cause problems that may require medical
intervention.
The Juicy Stuff: The "Sex Hormones"
The hormones commonly considered "sex
hormones" in the body are testosterone,
estrogen, and progesterone. Testosterone is
often referred to as a "male" hormone, and
estrogen and progesterone are often referred to
as "female" hormones. However, it is interesting
to note that no exclusively "male" or "female"
hormones have been identified. All hormones
characterized to date are present in all people
regardless of sex, as are the receptor
mechanisms that respond to those hormones.
In fact, the physical observation of the two
sexes we call men and women is the result of
differences in the amounts of individual
hormones in the body and differences in their
patterns of secretion (first in utero and then
again during puberty) rather than their presence
or absence.
In other words, testosterone, estrogen, and
progesterone are all produced by men and women,
but in differing amounts and in different
patterns.
"Steroids"
Endocrinologists classify sex hormones as
being in the family of "steroid
hormones"-derivatives of cholesterol that are
synthesized mainly by the gonads and, in smaller
amounts, the adrenal gland.
Steroid hormones are typically classified
into five groups of molecules, based primarily
on the receptor to which they bind:
- Androgens, such as testosterone -
Estrogens, such as estrodiol and estrone
- Progestins, such as progesterone
- Glucocorticoids, such as cortisol
- Mineralocorticoids, such as
aldosterone
For the purposes of this section, only the
androgens, estrogens, and progestins are
considered, as they are the hormones mainly
responsible for the development "secondary sex
characteristics," a term further described
below.
Primary and Secondary Sex
Characteristics
"Primary sex characteristics" refer to
physical characteristics present in the human
body that are directly involved in reproductive
function: namely the gonads and their accessory
structures. The development of primary sex
characteristics happens to the fetus in the
womb.
"Secondary sex characteristics" refer to
physical characteristics that are typically
associated with males/men and females/women but
are not necessarily related to reproductive
function.
Examples would include facial hair growth and
deepening of the voice in men, and growth of
breasts and increased fat deposits around the
hips in women. The development of secondary sex
characteristics usually begins at puberty, as
the levels and patterns of secretion of the sex
hormones in the body begins to change at that
time.
The androgen testosterone (and its derivative
dihydrotestosterone [DHT]) is
responsible for producing masculine secondary
sex characteristics such as facial hair growth,
deepening of the voice, increased body hair
growth, and increased muscle development.
Estrogen and progesterone play a vital role
in the menstrual cycle in female bodies.
Estrogen is also mainly responsible for
producing secondary sex characteristics such as
breast development, and increased body fat
deposits around the hip and thigh areas
The
production of sex hormones in the body
Testosterone, estrogen, and progesterone are
produced mainly in the "gonads" (the testes and
the ovaries).
Two other important hormones - Luteinizing
hormone (LH) and follicle-stimulating hormone
(FSH)- stimulate the gonads into secreting the
sex hormones. LH and FSH are secreted from cells
in the anterior pituitary gland, and are called
"gonadotropins" because of their role in
stimulating the gonads.
So What does all this mean?
For men born with typically "female bodies"
as well as men living with intersexual
conditions (eg. transsexualism), the goal of
testosterone therapy is to induce the presence
of masculine secondary sex characteristics. This
is done by introducing synthetic testosterone
into the body, thereby activating the androgen
receptor sites on target cells, which then
induce numerous masculinizing physical
changes.
Over time, as significant and regular doses
of testosterone are introduced to the body, the
hormonal balance in the body is altered, as are
the regular feedback patterns that control
estrogen, progesterone, and the fertility
cycle.
Therefore, testosterone therapy also usually
results in an eventual stop to the monthly cycle
and an interruption of fertility.
Over time with testosterone therapy,
fertility can be interrupted permanently.
For individuals who are post-menopausal, who
have otherwise nonfunctioning ovaries, or who
have had surgical removal of the ovaries,
testosterone therapy will induce masculinizing
secondary sex characteristics while not having
to "compete" with the estrogen/progesterone
hormonal production and feedback that is present
in those with active ovaries.
Younger individuals
In addition to testosterone therapy, other
hormone treatments could be considered for
treatment. For example, if an individual is
being treated for gender dysphoria at a young
age and has not yet undergone his first female
puberty, the onset of that puberty could be
delayed through "non-permanent" hormonal
treatment.
By treating a young (FTM) male with drugs to
suppress gonadotropin secretion (which work by
blocking the GnRH receptor) - or
'puberty-blockers' - his puberty could be
temporarily delayed while he considers his
options for further treatment.
Such a treatment may provide a young person
time to make a responsible decision about
starting androgen therapy, without the stress of
irreversible female effects on his body, as many
of the effects of 'puberty-blockers' are
reversible.
Whether this type of treatment is available
to the younger individual will depend on the
regulation and availability of puberty
blockers.
Changes to the original text made with
permission © 2004 Hudson's FTM Resource
Guide
This article also at
http://www.ftmguide.org/hormonebasics.html
