Hi Welcome to HCM - Have a great day I guess you are suffering from Hyperthyroidism or thyrotoxicosis,where due to overactive thyroid leads to increased metabolism and involuntary weight loss. I hope your doctor has put you on drug therapy or radioactive iodine therapy. Ask your doctor to adjust the dosage which will help you gain some weight. First you should try taking some thyroid inhibiting vegetables like Cauliflower, broccoli , Beans ,Green leafy vegetables and soy. 2nd Eat plenty of healthy fats like egg yolks, meat with animal fat, coconut oil. Increase caloric intake - of the healthy kind of course. 3rd Never let your protein intake go lower than 1g/lb of body weight when you are aiming to add long-term muscle. It s the building block of muscle, and your body is going to be starving for it. 4th - Take plenty of fruits - One banana contains approximately 100 calories. Not only are bananas rich in carbs and nutritious, they re also great for workout fuel. 5th - Oil - Adding extra fat to your food is an easy way to add calories, choose olive oil, canola oil, etc. which is good for your health and will add those much needed calories too. You can also try taking Protein Powders which gives extra proteins Also take Cheese / Butter which will help you gain some weight. I hope this helps, Dr Sumit Mazumder