Some obesity is directly related to your thyroid as well as other health issues, such as Hypothyroidism with makes most gain weight while Hyperthyroidism is the opposite.
Having said that my aunt who used to be as big as a bus (Hypothyroidism), lost over 280lbs and became a hotty and was also on Dr. Adkins infomercials about 10 years ago. She has since had a kid and ballooned back into a baby whale.