I'm 71, been on hypothyroid meds (Levothyroxine) for 25 years and haven't noticed any loss of muscle mass. I offset age-related loss of muscle mostly by weight lifting and other resistance exercise. Gardening and woodworking also help. If you don't currently lift weights then it would be good to get an experienced personal trainer because technique are critical for strengthening muscle groups. I started weight training 35 years ago. Resistance training is also very important for retaining bone mass (minimizing risk of osteoporosis). Regarding diet, most people get all the protein they need and it won't add muscle unless one is working out muscles. Creatinine is not recommended for someone with kidney disease, for example. Mediterranean and similar diets are better for us than most the processed food out there.