Just getting off T3 atm. it is safe to take as long as you are smart with it. Start at a low does, either 12.5 mcg or 25 mcg, taper up from there. Max I went to was 75 mcg per day, and then taper back down. Personally, I did:
25 mcg/day week 1
50 mcg/day week 2
75 mcg/day week 3
50 mcg/day week 4
25 mcg/day week 5
Worked great. Stacked it with Clen for 2 weeks, then with Hot Rocks for 2 weeks, cycling between the clen and Hot Rocks. There are a ton of posts about this, just dig on the boards a little.