I know many people who read your posting "how dare she think this is a problem" but believe me it is. I have the same problem and it can be quite dangerous,
Maybe you should have the thyroid rechecked. "A few years" is a long time. There are actually at least three thyroid tests, T3, T4 and a new one which is anti-thyroid antibodies.
Also, weight loss is a sign of diabetes (adults can get Type 1 diabetes, I unfortunately am living proof of that) so get your blood sugar tested as well.
I found for me that the best way to keep my weight up believe it or not is exercise. When I exercise, I eat more, and you just have to make sure that you are eating more than you are burning off. Also, muscle weighs more than fat, so there is a secondary effect as well. It doesn't have to be hours and hours, 3-4 trips to Curves per week worked for me. I injured my knee 9 months ago and stopped exercising (just had surgery last month) and yep, dropped 10 lbs. I know this is the opposite of what works for everyone else (i.e. most people exercise to lose weight, not gain it) but it works for me, and might work for you.