Re: Hidden causes of continual weight gain?
I would also continue to look into getting more detailed thyroid testing. Often times, a thyroid condition is NOT found through routine blood work and everything will look normal, so they need to perform some more specialized testing. An underactive thyroid (hypothyroidism) can definitely affect your weight if you are sure your eating/exercise/activity levels haven't changed. Yes, it's normal to gain some weight with age if you eat the same (you typically need less calories later on), but it does seem that this is pretty rapid.
I'd definitely get your thyroid looked at a little bit more FIRST. Also, see if you have any of the other symptoms of hypothyroidism. Good luck and I hope you are able to find out what's going on!