GtgtDr. 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 thyrotropinreleasing hormone. Thyrotropinreleasing hormone is abbreviated TRH. Once the neurosecretory neuron synthesize and secrete thyrotropinreleasing hormone, it will travel down the axon of the neurosecretory neuron and into the axon terminal.
From the axon terminal, thyrotropinreleasing hormone will be released. Once thyrotropinreleasing 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 thyrotropinreleasing 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 thyrotropinreleasing 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 thyroidstimulating hormone. So once the endocrine cells have synthesized and secreted thyroidstimulating 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 thyroidstimulating 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.
Human Physiology Thyroid Hormone Feedback and Function
Once it binds to those receptors, it’s going to promote the synthesis and secretion of thyroid hormoneTH. 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. Wefor the purposes of our classwe’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 thyrotropinreleasing 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.