I wrote here yesterday how we go through puberty and become imprinted with whatever baggage we are carrying at the time which means that your sexuality has been tied to your draw towards the feminine. Hence if we remove testosterone from the equation the libido is dampened thus helping to soothe the dysphoria but also diminish the need for feminization which seems counterintuitive right? But it's not really since your dysphoria, exposed to a cold shower of estrogen, has had its sexual energy greatly diminished.
With a lower level of dysphoria the person can now decide where they reside on the spectrum but they risk being feminized physically beyond a point they are willing to go. For some people a small amount of HRT is what they need to keep their dysphoria in check without entering into a full blown transition.
When I read Susan's (from transitioning into tomorrow) post it was a natural follow up to my post from yesterday. She is taking HRT and with the reduction in testosterone her need to express as female has been diminished. This doesn't mean one isn't transgender only that you may have found a balance point for your dysphoria.
So in a sense, if you want to be sure if transition is really for you then simply take HRT and it will give you your answer.
Stopping the administering of HRT can bring a return of dysphoria so knowing your limit becomes important. The article that Susan points to in her post from Anne Vitale provides some very useful information.
We know the general rules, but there's no way to know how your particular body and mind will react under HRT. What is recommended is that it should be administered and monitored by a licensed physician.