Med-Surg Endocrine System, part 6: Negative Feedback Mechanism vs. Positive Feedback Mechanism

by Cathy Parkes August 01, 2020

Full Transcript

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. So 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.

Alright. So 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. So 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. So 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. So 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. So 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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