A thoughtform is an energy construct, any energy construct. Generally when referred to in this context a servitor is intended. A servitor is a specific form of thoughtform which is semi-conscious/semi-sentient and is created for a specific purpose or set of purposes. In order to keep the servitor in existence (and thereby able to do it's predetermined job) you must figure out a way to "feed" it energy.
In most cases an outside energy source is used, as most practitioners of this form fear their servitors becoming energy leeches, turning on their creators, and hijacking their energy system. The servitor essentially becomes a parasite and if they are smart/strong enough, a flea bath won't kill them (read as trying to cleanse yourself of them).
That's the fear at least.
If the practitioner intends to feed it their own energy, you are better off limiting it's access to your energy (such as I put your energy into this stone, you take it only when you need it) or otherwise preventing it from being "over fed". That isn't to say you should let them starve either, as they may try something risky or try to go to another energy source.
However all of this is maybe's and might be's, since it is entirely dependent on how the servitor was made and whom it was made by.
I've personally never had an issue, but the way I work with my servitors is vastly different from how most people I know have worked with them.