As a woman who suffers from Hypothyroid, PCOS and Insulin resistance I can honestly say that these issues are not solely to blame for middleaged weight gain.
If I am gaining weight (fat weight) it's because I'm eating too much crap and not exercising and/or sleeping enough...period!
These disorders were a wake up call for me and started around age forty. I have had to adapt and find a healthy balance in order to maintain my weight and fitness level.
Yes it's much harder these days but I have found that If I push too hard, my body pushes back, and that took some getting used to. So I take my thyroid meds daily, tailor my workouts around where my body is today and get enough sleep and H20. My diet is as clean as I want it to be, and if it's not clean enough...it shows!
My hope is that women (and men) who are afflicted with these disorders would not use them as an excuse for their weight gain but rather gain control over how they eat and move. Gone are the days when I could use less discretion around food and just train much, much harder. If I did that now I would just get hungrier and be exhausted (and grouchy). Nowadays It seems as though the kinder I am to my body the more it allows me to continue to ramp it up in terms of training.
A Good Endocrinologist is a (midlife) gal's best friend