Thyroid v. "aging metabolism"

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OK - there's no health board here - so I picked fitness thinking it was the best fit.

Anyone finding that one day they just couldn't lose weight? I know it does happen but I'm a little freaked out by it. Haven't had my thyroid checked and not that I want meds or to have problems with it - but would be better (in my mind) than an "aging metabolism" where it just slows as you age (I'm only 35!).

I am working out WAY MORE but I'm not losing. Getting frustrated and I actually care less about physical looks than I ever have but to be gaining when I'm putting in so much more time.

Have you had problems in this area? What were your signs? What did you do?