As long as you are keeping your system balanced, it should be a problem. If your thyroid is slow enough, medication will keep TSH levels within proper tolerance.
Even with proper medication, your metabolism will likely be below average, and does make it harder to burn off fat, but shouldn't affect your ability to gain muscle mass, and sometimes makes it easier to retain muscle mass, since your system won't metabolize it for energy as easily as someone with a fast metabolism.
I have had hypothyroidism since birth, I take medication for it, and I haven't let it beat me. I'm quite able to gain muscle mass, and can burn off fat when in cutting phases, it just takes more work, so there is hope. It's only an obstacle if you let it control you.