I don't know what the percentage of people would be. However, I know that for me, I was hyperthyroid for untold number of years. I could lose weight almost by just thinking about it. Heck, I would walk 3 miles in the evening and then come home and have a huge serving of strawberry shortcake - and still lose weight! I just thought all these people who had a problem losing weight were weak-minding and had no willpower.

Then I got very ill and was diagnosed with hyperthyroid. My thyroid was "nuked" and after about 3 months I became hypothyroid. Suddenly, I was barely eating but gaining weight at a phenomenal rate. I gained more than 10 pounds in one month alone. Even after I was placed on hormone replacement therapy, I still found that I gained weight far more quickly than I ever had. My endocrinologist finally sat me down and had a discussion with after I had gained 30 pounds. I had to learn that I was now a person with a "normal" thyroid and I had to begin to learn to eat right. I could no longer just eat what I wanted and still lose weight. It was a major shift for me.

There are people (women, it seems to me more specifically) who have undiagnosed thyroid problems later in life. They suddenly begin to gain weight, even though their input of food is not that much more than it had been before. Sometimes these women experience other symptoms that allow for testing and diagnosis. Most times not. But, this is a situation where weight gain is a medical condition - hypothyroid causes your metabolism to slow down - and not one of simply overeating. So yes, hard as it might be for you to realize, weight gain (even dramatic gain) can be accomplished without overeating.