What can i do to build muscle when i have an overactive thyroid?

Treat thyroid!: As long as you have high thyroid levels, your body is breaking down tissue, including muscle, faster than you can build it. Depending upon how high the thyroid levels are, exercising may be dangerous. So fix the thyroid, get the thyroid levels normal, then work with weights or other forms of resistance training and make sure you're getting plenty of protein in your diet.