When exogenous Testosterone any type of steroid derivative is used, a bunch of actions take place ..
Your Hypothalamus (This is your master gland in your brain) secretes GnRH, that causes your pituitary gland to secrete LH & FSH. The increase in these hormones causes the testes to stimulate the Leydig cells to produce testosterone (by conversion of cholesterol). Testosterone then has the ability to undergo various metabolic processes that will inhibit GnRH, which in turn inhibits the secretion of LH and FSH, and that brings a halt to natural testosterone production. This is referred to as the negative feedback loop. Once testosterone has stopped being produced, it no longer sends this negative signal, and GnRH eventually begins to do its job again.
This is how homeostasis is maintained in the human body
then there is the estrogen aspect , as a lot of steroids convert to estrogen in the body and this signals for stopping producing test
""Aromatization mediates testosterone's short-term feedback restraint of 24-hour endogenously driven and acute exogenous gonadotropin-releasing hormone-stimulated luteinizing hormone and follicle-stimulating hormone secretion in men"" (from a study). The increased estrogen sends signals that there is to much conversion, while your Hypothalamus sends signals that too much estrogen is in your system, so the secretion of LH and FSH is halted. The leydig cells stop producing test, until homeostatis is met, and testosterone has a higher ratio to estrogen than it currently has.
^ this is also why its important to take an AI after your done with your cycle going into your PCT (but not during pct). you don't want elevated estrogen levels when you trying to recover natty test production. the estrogen will keep you shut down
so why do you need to take exogenous testosterone while running Deca. because your natty testosterone will be shut down and you will be without test, which is an important male sex hormone that does much more then work on androgen receptors in muscle cells. without test your sex hormone receptors will be inactive , you'll have no libido and no ability to get an erection, among other things.
