Would it be safe to go on a 800 calorie a day diet? i've put on weight as a result of my graves' disease.

Depends on...: You current height and weight. But that is typically a very low calorie count. Whereas you'd likely lose weight doing it, it's be hard to do, you wouldn't feel good doing it, and you would put your body in to starvation mode, slowly your metabolism down enough so that once you begin eating even a reasonable number of calories later, you'd regain the weight quickly. See nutritionist instead.