Thyroid hormones.
Tyrosine or T4.
And triiodothyronine which is T3.
When we want to think about the hypothalamic
pituitary thyroid axis we need to start in
hypothalamus.
In the hypothalamus we have two structures
that we need to take into account.
The paraventicular nucleus here and arcuate nucleus
which is seen below.
These are the places in which we are going to be
able to secrete the substances the thyrotrope.
So paraventicular nucleus secretes somatostatin
and arcuate nucleus secretes thyroid releasing hormone.
These are going to then be placed within
the circulation of the anterior pituitary.
And then the thyrotropes are what are going to
respond to either somatostatin or TRH.
So if we start off with the hypothalamus,
we have TRH.
That is going to be released to stimulate
pituitary thyrotropes.
But remember you have somatostatin which acts as
an inhibitor to those very same thyrotropes.
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.
The thyroid gland will then produce T3 and T4.
There is negative feedback with T3 and T4.
The first portion of the negative feedback,
is that it inhibits both the anterior pituitary
thyrotropes to release thyroid stimulating
hormone.
And it also inhibits the stimulatory nuclei.
And what this means is that even though thyroid
releasing hormone normally is released to stimulate
thyrotropes, you can't inhibit that nuclei
so it doesn't release thyroid releasing hormone.
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.
So what you do is you stimulate the inhibitor
or somatostatin to be released.
This acts as a nice negative loop
on multiple levels.
Both along the hypothalamic and the
pituitary level as well as effecting
both the inhibitor and the stimulator.
There are few other things we need to discuss
with T3 and T4.
So the most important one is, is that only
the free T3 or T4 participate in this negative feedback.
What do I mean by that?
The majority of T3, T4 is bound up with proteins.
In fact about 99% of it is bound up.
So you only have about 1% of your T3, T4 that is
actually circulating in the free form.
So you always have to know where you're at in terms
of where that 1% is.
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.
It's simply not being bound as well anymore.
So where does thyroid hormone come from.
It comes from the thyroid gland.
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.
These follicular cells are going to be the ones that
are going to be actively producing T3 and T4.
There are lot of blood vessels in the area.
And the blood vessels will do a couple of things.
One is it is going to deliver substrate that is
needed for thyroid hormone synthesis such as iodine.
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.