3-tier system of hormones

In this post, I will get into closer examination of endocrine or hormonal system. It's called 3-tier, because the secretion of hormones comes in three sort of "waves". The first one comes from hypothalamus, which release so-called relasing hormones, or in some older literature you can see it by the name releasing factors. Apart of those, there are release-inhibiting factors, which are hormones that act as having an opposite effect to releasing hormones.

 

Hypothalamus releases hormones into two ways. Either through the pituitary stalk into anterior pituitary lobe, or straight into posterior pituitary lobe. And those two lobes are already second wave or tier, from which series of another hormones are secreted. That would be ADH and oxytocin from posterior pituitary lobe and TSH, FSH, LH, PrL, GH, ACTH, beta-lipotropin, and beta-endorphin enkephalins from anterior pituitary lobe. All those hormones are directed towards gland or other organs, which become third tier in the system, and react to the signal by further secreting of another set of hormones.  

 

Those glands and organs could be thyroid gland or mammary gland, or liver, or nerves. For better illustration of what I have just written,  here is a picture I took from Open University course book on Human biology.  It summarizes all pathways I mentioned in nicely color flow diagram. In this case another proper example of human body flow diagram.

 

 

 

So let's start nicely to explain main hormones.

ADH is anti-diuretic hormone, which gives signal to kidney to start reabsorbing more water. When the level of water in the body is adequte, ADH will stop coming to kidney, because hypothalamus will stop releasing first set of hormones. I'll bring nice flow diagram in some future post, as this particular negative feedback deserves more attention in a separate post.

 

TSH stands for thyroid-stimulating hormone, which clearly means that thyroid gland will become stimulated for prodution and secretion its hormones thyroxin and tri-iodothyronine. Those than go out and have the effect on their target cells.

 

FSH is follicle-stimulating hormone, which means the follicle in the ovary and testes. Those produce and secrets their own hormones. For testes it's testosterone and for ovary it's progesterone and oestrogen. Those two organs are further stimulated by LH (luteinizing hormone), and ovary also with PrL (prolactin), which affects mammary gland too (for producing a milk).

 

Then we have here a growth hormone (GH), which gives signal to liver that it's time to build some more tissue - hence the growth promotion. Liver reacts by releasing IGF-1 (insulin-like growth factor), which starts the action by bringing more glucose from bloodstream into the cells, where it can be used as energy for building new tissue, or repairing the old one.

 

ACTH stands for adrenocorticotropic hormone, because it stimulates adrenal glands, and it further release the group of hormones called corticosteroids. So that would be all abbreviations from the picture above, but there is still more to come as I didn't mention no examples or releasing hormones yet. So here it is. There are two types of hormones coming from hypothalamus to anterior pituitary lobe, also called neurohormones:

 

Releasing hormones (or factors):

GnRH - gonadotropin-releasing hormone

CRH - adrenocorticotropic hormone-releasing hormone

TRH - thyroid-stimulating hormone-releasing hormone

GRH - growth hormone-releasing hormone

 

and release-inhibiting factors:

SIF - somatostatin - inhibits secretion of GH and TSH

(also known as GIF - growth hormone release-inhibiting factor)

PIF - prolactin release-inhibiting factor

 

So now I hope you have some idea about what hormonal system is. Of course, there is much more to be said about any particular hormnone or pathway, but that would be loads of pages about each one. This post serves for a rough summary, accompanied with nice flow diagram for better imagination of what is being written. Only time will tell if I expand this post in the future, but so far I think that it's deep enough. See you later.