If you have a lot of T3 in your body, you will easily lose both fat and muscle and look like a 17 year old who has never weight-trained. This is similar to "ectomorphs" who "can't put on weight", with the exception that some people just don't eat enough.
Your adaptation to dieting depends on your setpoint, current weight and genetics. Some people will slow down if they diet agressively, while others will not. Everyone has a particular hormonal reaction to a caloric deficit, and not everyone is the same.