A hormone will only act on a part of the body it 'fits'. A hormone can be thought of as a key, and its target site ( i.e an organ) has specially shaped locks on the cell walls. If the hormone fits, then it will work.
The hormone can set off a cascade of other singling pathways in the cell to cause an immediate effect ( for instance, insulin signaling leads to a rapid uptake of glucose in muscle cells)
The endocrine system is a tightly regulated system that keeps the hormones and their effects at just the right level. One way this is achieved is through ' feedback loops'. The release of hormones is regulated by other hormones, proteins or neuronal signals.
The released hormone then has its effect on other organs. This effect on the organ feeds back to the original signal to control any further hormone release.
btw- found all this info @ the Better Health channel, an australian government health website , so if your still confused by my answer, check out this website
