system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how did scientists discover different hormones in our body given that hormones are just tiny particles and how did they study it to know their purpose?
gpt
Hormones are secreted from different parts of the body, if those parts are damaged or removed the hormones they produce are no longer made. One of the earliest observations of this was in the 1840’s when scientists noticed animals lacking testes had different sex characteristics than other males, and ultimately discovered testosterone. 

Other hormones where discovered in similar ways. Scientists noticed that removing the pancreas from a dog will give it type 1 diabetes, they then realized that grinding it up dog pancreas, purifying it and injecting it into people with diabetes relieved their symptoms (and was a big deal as diabetes used to be an agonizing way to die, the only treatment was starvation). 

Adrenaline/epinephrine was isolated from the adrenal gland (they sit above your kidneys) and was incredibly useful for its ability to constrict blood vessels, particularly to stop bleeding during precise/fine surgical procedures.