by Cathy Parkes August 05, 2020
Negative feedback mechanisms and positive feedback mechanisms are important concepts to keep in mind throughout your Med-Surg class. Knowing how these work helps you understand how different hormones respond to one another. If you know how the body processes are supposed to work, you can identify and diagnose when something goes awry in one of those processes.
When confronted with an exam question about a disease you may not remember the exact symptoms or labs for, if you remember what the hormones are supposed to do and how they are supposed to respond to one another, you can deduce those answers.
A negative feedback loop is one way that the endocrine system tries to keep homeostasis (stability) in the body. If an endocrine gland senses that there is too much of one hormone in the body, it will initiate changes to decrease production of that hormone. And if there’s not enough of the hormone, the body will increase production of that hormone. You can think of it as your body’s attempt to self-regulate and self-correct.
If you have heating in your home, you can set the thermostat to a certain temperature and then your heating systems will kick on to maintain that temperature.
If you set the thermostat in your home to 75°, your heat will run until your temperature gets to 75°, and then it will turn off. If it gets cooler in the room and drops below 75°, your heat will turn back on to return the room to 75°, then turn off once it succeeds. This will continue to happen so long as your thermostat is set to 75°.
The endocrine system works similarly to the thermostat, but with your hormones. The 75° represents your body’s preferred levels of different hormones.
The hypothalamus produces thyroid-releasing hormone (TRH). This causes the anterior pituitary gland to produce thyroid-stimulating hormone (TSH), which causes the thyroid gland to produce thyroid hormones T3 and T4.
But if the hypothalamus can sense that there is excess T3 and T4 in the body, it will decrease production of TRH. This causes the anterior pituitary gland to decrease production of TSH, which causes the thyroid gland to decrease production of T3 and T4.
The anterior pituitary gland can also be the one to sense if there’s too much T3 and T4 in the body. If it does, the hypothalamus/TRH step is skipped, and the anterior pituitary gland decreases production of TSH, which then causes the thyroid to decrease production of T3/T4.
Both of these scenarios work the opposite way as well. If there is not enough T3/T4 in the body, the hypothalamus will increase production of TRH. This causes the anterior pituitary gland to secrete an increased amount of TSH, which causes the thyroid to increase production of T3/T4. This is how homeostasis is maintained in the body.
If there is a problem with the hypothalamus, anterior pituitary gland, or thyroid, then it can cause a problem with the whole system.
For example, in primary hypothyroidism, there’s a problem with the thyroid (often from Hashimoto’s disease). If the thyroid gland is damaged, it can’t play its normal role in this negative feedback loop, and can’t produce T3 and T4 like it should.
In secondary hypothyroidism, there’s an issue with the pituitary gland. This decreases the production of TSH, so the thyroid gland doesn’t get the message to produce T3/T4.
A positive feedback loop in the endocrine system is more rare, happening for only some hormones. A positive feedback loop in the endocrine system is when release of a hormone initiates actions that lead to an additional release of that hormone. Unlike a negative feedback loop, a positive one is not looking to reach homeostasis (stability).
Oxytocin is one of the few hormones regulated by a positive feedback mechanism. In both childbirth and breastfeeding, oxytocin is released and causes additional release of oxytocin.
During childbirth, release of oxytocin results in uterine contractions, and uterine contractions cause additional oxytocin to be released. Release of oxytocin then stops after childbirth.
During breastfeeding, oxytocin is released, which allows for milk ejection. Milk ejection causes more oxytocin to be released. Release of oxytocin stops when breastfeeding stops.
Understanding these mechanisms and how they work is crucial to understanding the pathophysiology of the related diseases and disorders of the Endocrine System. The Medical-Surgical Nursing video series is intended to help RN and PN nursing students study for nursing school exams, including the ATI, HESI and NCLEX.
Okay, in this video we are going to talk about the negative feedback mechanism that controls levels of most hormones within the endocrine system, and then we'll also touch on the positive feedback mechanism that controls levels of a few of the hormones within the endocrine system.
So let's first talk about the negative feedback mechanism, which works similarly to your thermostat.
If your heater is turned on in your home and your temperature is set at 75, your heater will run until that temperature gets to 75. Then it will turn off. As the temperature lowers when it goes below 75, it will turn back on, and it turns off and on to keep the level right there at 75 degrees.
Well the hormones within the endocrine system, it kind of works the same way. If an endocrine gland senses that there is too much of a circulating hormone in the body, it will initiate changes to decrease production of that hormone, and if an endocrine organ or gland senses there's not enough of a circulating hormone, it will initiate changes to increase production of that hormone, to try to keep homeostasis, just the right amount of hormones circulating in the body.
Let's now look at a a specific example of that negative feedback mechanism or negative feedback loop. We're going to look at the release of thyroid hormones, which are T3 and T4.
Starting off, the hypothalamus produces thyroid-releasing hormone which causes the anterior pituitary gland to produce thyroid-stimulating hormone, and then this causes the thyroid gland to produce T3 and T4. And it releases T3 and T4 into the body, where it goes and does its job.
So if the hypothalamus senses that there is too much T3 and T4 out there, it will decrease production of TRH, which will, in turn, cause the anterior pituitary gland to decrease production of TSH, and this will cause the thyroid gland to decrease production of T3, T4.
And that is what we mean by a negative feedback mechanism.
The anterior pituitary gland also can sense if there's too much T3 and T4 circulating in the body, and if it does sense that, it will, by itself, decrease TSH, which will, in turn, decrease production of T3 and T4 by the thyroid gland.
This works kind of the opposite way as well. So if there's not enough T3, T4 circulating in the body, the hypothalamus will increase production of TRH, which will cause the anterior pituitary gland to increase production of TSH, and that will cause an increase in production of T3 and T4 by the thyroid gland.
This how we maintain homeostasis for these thyroid hormones. However, if we end up having an issue in the hypothalamus or in the anterior pituitary gland or in the thyroid gland, then that really throws a wrench in everything. So when we are talking about hypothyroidism and hyperthyroidism, we will get into more detail about that.
Okay. So that negative feedback loop example that I just explained, that controls the release of most endocrine hormones.
However, there are a few hormones that are controlled by a positive feedback mechanism instead. So with a negative feedback mechanism, like we just talked about, if we release a bunch of hormones, the body will sense that, right? The organs and glands will sense that, and they will decrease production of that hormone. That's a negative feedback loop.
With a positive feedback mechanism, when we release a bunch of hormone, that causes additional release of that same hormone.
So oxytocin is a hormone that is regulated by a positive feedback mechanism. And I'll give you a couple of examples where oxytocin is released and that the release of oxytocin causes additional release of oxytocin.
One is during childbirth. So release of oxytocin allows for uterine contractions, and uterine contractions cause additional oxytocin to be released.
Also, we see it with breastfeeding. So breastfeeding causes the release of oxytocin, which allows for milk ejection. And this in turn causes an additional release of oxytocin, so it just compounds that. So oxytocin will stop being produced after childbirth or after a mom is done breastfeeding. And that's how production of oxytocin does end up decreasing.
So hopefully, that was helpful in explaining the difference between negative feedback mechanisms and positive feedback mechanisms. Stay tuned. We'll go over more important endocrine information!
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