What kind of specialist should i go to for weight loss?

i have about 40 pounds to lose and it's proving to be very hard to lose. i feel ive always eaten the same way and gained this amounf of weight on only 2-3 years and im only 25. ive had some other random health problems that may or may not be connected. whenever i go to my general practicioner she's like there's nothing wrong with you and that i am not taking care of myself and not a kid anymore. she may be a bit right but I would still like to see if there are underlying causes to this. nutritionists just create menus and chart your weight loss progess, so what kind of doctor would i go to for this? i am not at all fat enough for stomache surgery and dont really believe in it anyway. i would like to get my metabolism/thyroid, etc. tested?