I can imagine how this information has caused such a high level of anxiety and it may seem callous that the doctor couldn't tell you more, but thyroid readings are tricky. It sounds like he needs to compare your current readings to new readings in 3 weeks to continue the investigation into what is going on. This doesn't automatically mean it's cancer. It is so hard to stop your mind from going there. I've done it too.
How are you feeling today? Has the increased dose of Synthroid made a difference yet?