Hello, This is Dr. Papaqako answering your question. Thank you for posting your concern on HCM. If your labs have come out normal, and you have been underweight all your life (so its not a new concern), I would reassure you that is nothing wrong with you, you just have a fast metabolism. I would recommend some things to make them a lifestyle, that may help: 1) Eat a healthy diet. Meaning, several meals during day, with main component the proteins, and less of carbs and fat, fruits and vegetables should be a main component of your meals, snacks in between meals, avoid smoking, drugs and alcohol. 2) Exercise regularly. Start with at least 20 min a day, for 5 days a week. Progressively you should add strength exercise, that will strengthen and make muscles evident in your body, which will result in weight gain. 3) Have patience. Will need at least 80 days to you see the result of your hard work in exercise and weight gain. by that time, gym will be your favorite time to spend for yourself. Hope this helps. Let me know if any other question. Take care.