I don't think you need to know your TPMT when stopping Aza/6mp. They use it to determine how you might metabolise the drugs when you do start taking them.
On the pred front, I have had a few stressful things on the last few days, one of them ending with something that finished last night, and already this morning I feel a bit better. I'm going on the theory (made up in my head!!) that the steroids, by overriding the body's own corticosteroid system, have caused a massive over-exaggeration of my stress response. As soon as I was thinking about the things going on I would get palpitations, sweating, ocular migraine, shaking, etc...
I think some serious relaxation techniques are in order!