I used to think I had a fast metabolism until I was diagnosed with the opposite, underactive thyroid which makes poeople put on weight. I have always been quite thin, 7 stone 7-11 for about seven years and my BMI was usually about 17.5. I do no exercise and never ever skip meals, I have a good hearty breakfast, lunch and dinner and snack and eat rubbish in between. When I am ill I lose weight bigtime and find it takes months to get back up to the level, but no further. 2 years ago I dropped from 7 stone 10 to 6 stone 4 and could barely move when I was in a lot of pain and couldn't eat properly- IBS diagnosis. My thyroid results are never 'normal', it's always under. Usually controlled well with meds so that it's not too bad but it flares up every year or so and makes me so tired I can't leave the house at all. I never gain extra weight though.
However I think that I *might* be turning the corner now I'm getting older, I'm nearly 24. I'm 8 stone 1 with a BMI of 19.8- normal range! Obviously still thin but no longer underweight, so I'm thrilled. I'm hoping it stays that way and doesn't go back under the 8 stone mark and hopefully in the future with children I will be heavier from that. Not that I would have kids FOR that reason of course, but it would be a nice bonus.