SlateDrake: You have it backwards.
Yes, levo is T4, but the body converts T4 into T3. That's the natural process. So in effect, T4 is safer, because it allows the body to still do what's t's supposed to do. (Even though it's an exogenous source and can still supress the thyroid.)
Hypothyroidism occurs when the body can no longer convert T4 into T3. That's what makes taking T3 so dangerous. The thyroid doesn't do ANYTHING when that much T3 is present. it assumes there is TOO MUCH and comes to a standstill. When you go off the T3, the thyroid might spring back into action -- it might not. It might take a while -- it might never come back. Taking T3 is playing with fire. For what?