Yes, there are a few folks out there with serious health issues that make it very difficult to maintain a healthy weight.
But we're even more in denial thinking they are a significant number of the obese. They aren't- they're convenient excuses. I can think back to my wife's roommate in college who was morbidly obese, even though she ate healthy meals and exercised by biking around campus. She explained to everyone that she had glandular problems that caused the weight gain. Those same glandular problems also apparently caused her to eat cans of frosting late at night when she was depressed or stressed. (Yes, you read that correctly)
My metabolism has changed over the years- I was rail thin through college no matter what I ate. After 25 years of post-college life eating the same food, I was pushing the obese range- my body's burn rate has slowed even though I exercise quite a bit. So I stopped eating so much crap last year. Metabolism change or no, I'm down 35 pounds and a hair away from "healthy" BMI. It might be harder to lose weight with a crap metabolism, but it's hardly impossible.