00:00
Thyroid hormones.
00:02
Tyroxine or T4.
00:05
And triiodothyronine
which is T3.
00:10
When we want to think
about the hypothalamic
pituitary thyroid axis
we need to start in
hypothalamus.
00:20
In the hypothalamus we
have two structures
that we need to take
into account.
00:24
The paraventicular nucleus
here and arcuate nucleus
which is seen below.
00:34
These are the places in
which we are going to be
able to secrete the substances the thyrotrope.
00:42
So paraventicular nucleus
secretes somatostatin
and arcuate nucleus secretes
thyroid releasing hormone.
00:50
These are going to then
be placed within
the circulation of the anterior pituitary.
00:56
And then the thyrotropes
are what are going to
respond to either
somatostatin or TRH.
01:01
So if we start off with
the hypothalamus,
we have TRH.
01:07
That is going to be
released to stimulate
pituitary thyrotropes.
01:13
But remember you have
somatostatin which acts as
an inhibitor to those
very same thyrotropes.
01:19
So if there is enough TRH
around and you're able to
allow those thyrotropes
to release their hormone,
which is thyroid
stimulating hormone,
that will then engage
the thyroid gland.
01:33
The thyroid gland will
then produce T3 and T4.
01:37
There is negative
feedback with T3 and T4.
01:40
The first portion of
the negative feedback,
is that it inhibits both
the anterior pituitary
thyrotropes to release
thyroid stimulating
hormone.
01:52
And it also inhibits
the stimulatory nuclei.
01:56
And what this means is
that even though thyroid
releasing hormone normally
is released to stimulate
thyrotropes, you can
inhibit that nuclei
so it doesn't release thyroid
releasing hormone.
02:12
Now in terms of the
feedback control, you also
have one more arm
and that arm positively
stimulates the hypothalamic nuclei
that inhibits the anterior
pituitary thyrotropes.
02:25
So what you do is you
stimulate the inhibitor
or somatostatin to be released.
02:33
This acts as a nice
negative loop
on multiple levels.
02:38
Both along the
hypothalamic and the
pituitary level as well
as effecting
both the inhibitor
and the stimulator.
02:48
There are few other things
we need to discuss
with T3 and T4.
02:52
So the most important one
is, is that only
the free T3 or T4 participate
in this negative feedback.
03:01
What do I mean by that?
The majority of T3, T4 is
bound up with proteins.
03:08
In fact about 99%
of it is bound up.
03:13
So you only have about
1% of your T3, T4 that is
actually circulating
in the free form.
03:20
So you always have to know
where you're at in terms
of where that 1% is.
03:27
If you have a reduction
in the amount of binding
proteins, you may look
like you have an elevation
in your T3, T4, but
you're not producing it.
03:36
It's simply not being
bound as well anymore.
03:42
So where does thyroid
hormone come from.
03:44
It comes from the
thyroid gland.
03:47
The thyroid gland if we
looked at the histology of
it, is broken out with
many of the cells wrapped
around what we
call a colloid.
03:55
These follicular cells are
going to be the ones that
are going to be actively
producing T3 and T4.
04:03
There are lot of blood
vessels in the area.
04:05
And the blood vessels will
do a couple of things.
04:09
One is it is going to
deliver substrate that is
needed for thyroid hormone
synthesis such as iodine.
04:18
It's also going to be
removing the T3 and T4, so
that it can then circulate
throughout the body
and do it's target
end organ functions.