Question: When do hormones know when to start and stop producing?

Keith Siew answered on 25 Jun 2013:

So there are lots of different mechanisms… but a good example is a system called “negative feedback”

So for example… testosterone… which is produced by the testes in males (and testosterone is also produced by the ovaries in females but not in the same quantities). Well this is involved in a signalling network called the “hypothalamic-pituitary-testicular axis”.

So when we need higher levels of testosterone the hypothalamus releases “gonadotropin-releasing hormone” (GnRH)… which stimulates the pituitary, the tiny peanut shaped structure at the base of the brain to release “luteinizing hormone” (LH) and “follicle stimulating hormone” (FSH) into the bloodstream…. these then travel down to the testes and instruct them to increase the amount of testosterone they produce.

So that’s the positive arm of the signalling network… to cause and increase… but mother nature is clever and to stop you getting inappropriately high levels of testosterone… testosterone itself has an inhibitor effect on the hypothalamus and pituitary! So when the levels of testosterone increase to a certain point they “turn down” the amounts of GnRH, LH and FSH being produced… in what we call a negative feedback setup. So this way you should have just the right amount of testosterone in your body… because each step of the signalling network/axis keeps the other in check in a circle of feedback.

That’s why when athletes dope with testosterone they get testicular shrinkage… because the excessive negative feedback of “exogenous” or doped testosterone together with your natural levels, over suppress the hypothalamus and pituitary causing sharp fall in levels of GnRH, LH and FSH. This causes the testes to stop “growing” and shrink in size to stop producing so much testosterone… because the body thinks the testes are the sources of the extra testosterone… not a needle point!