T3 (thyroxine triiodothyronine, Liothyronine) and T4 (thyroxine tetraiodothyronine, Levothyroxine) are naturally produced thyroid hormones in the body that play a key role in regulating metabolism. Bodybuilders sometimes use a combination of these two hormones to speed up fat burning and increase metabolic rate, resulting in clearer muscle mass during competition preparation or fat loss. …