Hi Bill! ...is your username as in "Bill the Cat"?
If you are asking if you can use thinset to prepare a floor by filling in some of the low spot, yes.......but it has its limits. Jim mentioned something that I don't entirely agree with:
Originally Posted by jadnashua
Let it cure overnight before you start to lay down tile in a new bed of thinset.
I would wait longer before tiling over thinset that was used the day before to prep.
Let's first find out how out of whack your floor is. Grab the longest straight-edge you have. Lay it on the floor and check the floor in many different areas. Now tell us how long the straight-edge is and how big the gaps are under the straight-edge. Is this a slab or joist floor construction?
What kind of tile are you considering?