>>Dr. Ketchum: In this segment we will learn how thyroid hormone is synthesized and secreted,
and how that synthesis and secretion can be turned off. So let’s begin with the hypothalamus.
These neurosecretory neurons will synthesize and secrete hormones, and these hormones can
travel to the anterior pituitary. In this case with the thyroid hormone feedback loop,
the neurosecretory neurons will synthesize and secrete a hormone called thyrotropin-releasing
hormone. Thyrotropin-releasing hormone is abbreviated TRH.
Once the neurosecretory neuron synthesize and secrete thyrotropin-releasing hormone,
it will travel down the axon of the neurosecretory neuron and into the axon terminal.
From the axon terminal, thyrotropin-releasing hormone will be released. Once thyrotropin-releasing
hormone is secreted from the axon terminal, it will enter into the hypophyseal portal
system. This hypophyseal portal system is basically a bridge between the hypothalamus
and the pituitary gland, specifically the anterior pituitary. So thyrotropin-releasing
hormone will travel through the hypophyseal portal system, this capillary bed, and into
the anterior pituitary. Once TRH reaches the anterior pituitary, it will stimulate or it
will bind to, in other words, to receptors on the endocrine cells within the anterior
pituitary. So once thyrotropin-releasing hormone binds to these receptors on the endocrine
cells in the anterior pituitary, that will stimulate the endocrine cells of the anterior
pituitary to synthesize and secrete another hormone called thyroid-stimulating hormone.
So once the endocrine cells have synthesized and secreted thyroid-stimulating hormone,
it will enter into circulation. And this is the systemic circulation.
So and it’s a hormone, so we know hormones enter into the bloodstream. So once thyroid-stimulating
hormone enters circulation, it is going to target the cells of the thyroid gland. So
once TSH targets the thyroid gland and targets those cells, it will bind to receptors. And
once it binds to those receptors, it’s going to promote the synthesis and secretion of
thyroid hormone—TH.
One of the things about thyroid hormone, there are actually two types. There’s a T3 and
a T4 that are actually synthesized and secreted by the thyroid gland. So we have T3 and T4.
We—for the purposes of our class—we’re going to refer to both T3 and T4 as thyroid
hormone, but let me just briefly explain the difference between the two. T3 is called triiodothyronine
because it contains three iodides; T4 is called thyroxine because it contains four iodide
molecules. T4 is more prevalent than T3, but it has a weaker effect. T3 on the other hand
is scarcer, but four times more potent. So both T3 and T4 wind up in circulation, in
the systemic circulation. When T4 winds up into the systemic circulation, it gets converted
to T3. And so T3 then will go off and will target various tissues.
So we have our thyroid hormone that’s in circulation, and the thyroid hormone does
a number of things. One of the things that thyroid hormone can do is it can increase
the number of mitochondria within the cells. Thyroid hormone can also target many different
tissues. And when it targets these tissues, what it’s doing is it’s controlling the
rate of cellular respiration, so in other words, controlling the rate of metabolism.
So we’re controlling the rate of cellular respiration, and this is going to be dependent
upon what the body’s needs are.
Now throughout the body, thyroid hormone can also increase the rate of your sodium potassium
ATPase. If you remember, that is the sodium potassium pump. So if I increase the rate
of my sodium potassium pump that means I’m going to increase the rate at which I hydrolyze
ATP. So all of these then contribute to increasing your basal metabolic rate. And if I increase
the number of chemical reactions within the body, my body is generating heat. So we generate
heat as a product of increasing our basal metabolic rate. So what this means then is
we are creating a need for more ATP. So to do that, I am going to need to mobilize the
energy stores. Now, when thyroid hormone functions to mobilize those energy stores, thyroid hormone
is at a slightly higher concentration than it normally would be.
So thyroid hormone will target the liver, thyroid hormone will target skeletal muscle.
And when it targets the liver and the skeletal muscle, that thyroid hormone will cause glycogenolysis.
And as a result of glycogenolysis, my plasma glucose level will increase. Now thyroid hormone
can also target adipose tissue. And when it targets adipose tissue, that’s going to
stimulate lipolysis. And as a result of breaking down a triglyceride, that’s going to increase
our fatty acid plasma level, and it will also ultimately increase the glucose plasma level
as well. Now thyroid hormone can also target skeletal muscle and cause proteolysis. And
when we break down muscle via proteolysis, then we’re going to be releasing amino acids.
So as a result, the body will also increase the plasma level of amino acids. All of these
then can be used as substrates for cellular respiration. And because they’re used as
substrates for cellular respiration, there is no net effect of your glucose level in
the plasma.
So we’ve discovered then with thyroid hormone function is the big picture is that it contributes
to growth and development. It also maintains a balance between your catabolic and your
anabolic reactions. So we’re going to be using negative feedback to turn off thyroid hormone
synthesis and secretion. If we start with thyroid hormone again that’s in the circulation,
thyroid hormone can have an inhibitory effect on the anterior pituitary. So when it has
this effect on the anterior pituitary, it’s inhibiting the responsiveness to thyrotropin-releasing
hormone. Now thyroid hormone can also have an inhibitory effect on the hypothalamus.
And when it binds to receptors within the hypothalamus, it will inhibit the synthesis
and secretion of thyrotropin-releasing hormone.