If you have health insurance you can get bloodwork done easy if your doctor is cool. A couple times a year I tell her I want it checked (she knows because of my supplmentation and she has told me the dangers of creatine - give me a fucking break), they send a form to go to a lab near my house and I run in and have blood taken and the results are in 2 days later. I usually need to specify what I want tested, - blood sugar levels, TSH, liver enzyme, kidney and a couple other things for good measure.
After doing T3 at 75-100mcg for 4 weeks straight my thyroid was still completely normal. I've done this at least twice to see how my thyroid was effected. I would be on 100mcg for at least 4 weeks and 50-75mcg for the other 2 weeks as a very quick taper up and down. This may not be the same for everyone though so I'm not saying that taking T3 at high levels won't effect you. It just doesn't seem to drop mine at all.
I never personally tell people to go above 50mcg because if they haven't done the research themself I don't want to be responsible for anything that may occur.