I suppose it’s when it starts to turn into an obsession, LadyBug, that it’s more an eating disorder than a desire to be thin. Also, if the desire to be thin is unrealistic (wanting to be TOO thin), that’s certainly an eating disorder trait.
What 2crazy4u stated are all sure signs of an eating disorder, so if you feel that you, or somebody else (?), are falling under several of those categories, you would certainly be best to go and get a professionals opinion as soon as possible. Don’t put it off if you suspect you have an eating disorder; the longer you wait, the more serious the eating disorder gets and the harder it is to recover.
Let us know if you need to know any more. Perhaps a search on the internet would be a wise idea so you can understand a little more about eating disorders.
Good luck!
Ashlee