I have been on those as well for years but have not experienced loss of muscle mass. It may be your diet - are you getting enough protein that is needed for muscles? I also have been adding creatine to my smoothies which is touted as helping. Overall, diet is incredibly huge to our health in many ways. I strongly recommend finding a nutritionist who is knowledgeable about health issues and recommending foods that will aid you in growing muscle mass. There are also foods to look at that are good for the and thyroid issues. It's finding the right person to help you with that. And your loss of muscle mass could very well be due to something else. Perhaps you are not absorbing the nutrients you eat from foods and supplements.