I Understand That Meth Isn't Health Food, But Wondering Why

I understand that meth isn't health food, but wondering why it is contraindicated - is especially harmful if you are taking synthroid for hypothyroidism. I started using before (and prob because of) extreme hypothyroid. It is now (with synthroid) normal, but i still use once in a while... but just a tiny tiny bit seems almost too much. Is it synergistic with the synthroid? or what? or am i just remembering doing it when my thyroid was soo low that it, then, seemed to make me feel "mormal".