Thesis statement writing is hard. They seem to be a bit of a paradox too because they need to be both broad and specific at the same time. Too broad of a thesis statement does not effectively guide the argument. Too narrow a statement chokes potential avenues of discussion out of the paper.
One of the first things to remember is that a thesis statement makes an argument. It can't say that anorexia nervosa is an eating disorder. That's a fact, and there is nothing to discuss. Make an argument with the thesis.
I'm not sure what your intended argument about anorexia nervosa is, but I can offer some general thesis-formatting tips. I generally recommend to students that they form a two-sided thesis statement. The first half of the statement admits to a particular argument that the second half of the statement works to either counter or further enhance. This kind of statement format generally guides the paper to be fairly well-rounded in the number arguments and discussions that it can highlight for readers. For example, the following statement might work well.
Although anorexia nervosa is one of many eating disorders affecting young teenage girls, it should be considered the most dangerous eating disorder.
That thesis allows the possibility for the paper to discuss other eating disorders. It then also gives you the ability to focus on how and why anorexia is so damaging to the body.