Most of the time people take t3, they start a strict diet. As we all know, diet makes us very week. Espesially low carb diets. So maybe that's where your weakness comes from. Just remeber that t3 burns more calories due to the increase in your metabolism/thyroid out put.. So if your diet didn't change at all, then your body is still geting less calories to use for energy because t3 speeds up your metabolism. When your metabolism is slower or normal, you don't use as much calories. The acsess calories get stored as fat.