Are you healthy? I am thin but not as much as I'd like (mental issue) and always have to remind myself that the result of a healthy lifestyle is a healthy body. Not one that necessarily looks like your ideal form. I can walk & dance, stand on my hands & cartwheel, eat healthy, exercise and work on athletic goals not size goals.

If you are happy with your lifestyle and feel like you are nourishing your body and exercising it, weight lands in different places for different people (or as I found, different for same person at different ages.) But you feel good? That seems like a good result.

If you don't feel good, that is when I would push the doctor for more tests to figure out what the heck is going on.