Hypothalamic-pituitary-thyroid (HPT) Axis and Hormone Cascade (Nursing)

00:01 The hypothalamic pituitary thyroid axis and the hormone cascade that goes along with it is what allows the body to maintain homeostasis.

00:10 Now let's start at the top.

00:11 Why does the hypothalamus produce TRH? That first step well the hypothalamus produces TRH, which is thyrotropin releasing hormone because it receives several physiological signals and realizes that there's a need for TRH.

00:28 Now let's look at four key triggers for the hypothalamus to recognize the body needs TRH. So the first one is it'll look at and measure and get signals about the client's energy status and their metabolism.

00:42 So when the hypothalamus notices there's a low metabolic rate, it's going to stimulate more TRH.

00:49 If the patient is exposed to cold and you need to increase heat production, the hypothalamus will know it's time to produce more TRH.

00:56 And also things like changes in energy expenditure needs and even blood glucose levels. So when we're talking about why the hypothalamus is stimulated to produce TRH, the first one are signals that come from the patient's energy status and their state of metabolism.

01:15 The second one is the circadian rhythm.

01:18 So TRH production follows this daily pattern.

01:21 It's usually highest in the early evening and lowest in the early morning. The third signal or environmental signals.

01:29 Remember temperature changes the light and dark signals, and a patient's response to stress.

01:34 The fourth one is metabolic feedback.

01:37 And I wanted to go into this a little more detail.

01:40 When the patient has low thyroid hormone levels, low T3 and T4 in the bloodstream, the hypothalamus, that's a trigger to put out more TRH.

01:50 Also, you can have changes in leptin and other metabolic hormones that will also stimulate TRH. And finally, the client's nutritional status plays a definite role as a trigger for TRH production.

02:03 So the hypothalamus acts as like a master regulator.

02:07 It takes all these various signals and it determines how much TRH to produce. Now, this is what ensures that there's an appropriate thyroid hormone levels that the patient needs to maintain optimal metabolic function.

02:20 This helps them grow, develop and monitor and regulate their temperature. So in order for you to get a clearer picture of how this axis works, let's start with that master regulator, the hypothalamus. So remember this is called the hypothalamic pituitary thyroid axis, the HPT.

02:40 So let's start at the H the hypothalamus.

02:43 So the hypothalamus senses we need more thyroid hormones in the body.

02:47 So it will produce thyrotropin releasing hormone okay.

02:52 Now it has a target. You know that hormones are chemical messengers.

02:56 They travel through the body and tell someone else another organ or tissue to do something. So the hypothalamus produces TRH.

03:05 TRH will travel through the hypophyseal portal system to the anterior pituitary. So hypothalamus produces TRH.

03:14 TRH goes to the target of the anterior pituitary gland.

03:19 Now when it hits the anterior pituitary gland TRH will bind to receptors on the thyrotroph cells.

03:26 Now these are special cells in the anterior pituitary.

03:29 Once this happens this will stimulate the production and the release of thyroid stimulating hormone, or TSH from the anterior pituitary, and it is released into the bloodstream.

03:42 So at the thyroid gland, this TSH that's in the bloodstream will bind to the receptors on the thyroid follicular cells.

03:49 Now this is going to trigger several processes.

03:52 It's going to increase iodine uptake.

03:54 It's going to increase thyroglobulin production.

03:57 And it's going to increase thyroid hormone synthesis.

04:01 So this all makes sense doesn't it.

04:03 The hypothalamus says wow we need more thyroid hormones.

04:07 It sends a message to the anterior pituitary.

04:10 The anterior pituitary receives that chemical message, sends a message to the thyroid gland to tell it to make more of these hormones.

04:20 That's why when we talked about hypothyroidism, when the patient isn't able to respond, when they have primary hypothyroidism, that body is shooting out TSH, but the thyroid gland can't respond.

04:33 That's why TSH pH levels are high in primary hypothyroidism.

04:38 After TSH binds to the receptors on the thyroid follicular cells, it's going to produce and release two things mainly T4, about 80%, which is also known as thyroxine, and some T3, about 20% of that. So keep in mind the thyroid actually produces significantly more T4.

04:58 So how do we get the T3 that the body needs? Well, it happens in the peripheral tissues.

05:04 So most of that T4 is converted to the more active T3 by specific enzymes. So this conversion happens primarily in your liver kidneys and in skeletal muscle.

05:16 So that wraps up the HPT axis.

05:20 And the following hormone cascade that helps the body maintain homeostasis.