I assume since you are in this group of 40 somethings, that is your age, and you are not in your 60s or older.
So I have to say, there is no reason for you to have muscle loss. There are all kinds of reasons that weight loss is more difficult at our age, but loss of muscle mass is not one of them.
You may have lost muscle, but thru lack for fitness, not aging.
SO you can work to get more fit and rebuild your muscle mass to help yourself lose weight. I don't know how out of shape you are, and I am sure there are lots of people who can give better fitness advice than me. (I have a long way to go, but for now am walking daily, doing yoga some days and have been doing pilates once a week. No weight training at this point)
As for going to the doc, I have to say, we all want to be diagnosed with a thyroid disorder so we can have an excuse for our weight problems, and can get a magic pill to help us lose. Its called denial.
I think its hard to accept, just like its hard to accept what we have to do to lose weight and get healthy.
I wasn't sure when I started coming here whether I was ready for the commitment to weight loss. I have been taking it one day at a time, and getting farther than ever before.
So be encouraged,
fatmad