To gain weight with hyperthyroidism, you should follow the same guidelines you would if you had a normal functioning thyroid with one exception – you will require more calories.
To gain weight, you need to consume more calories each day than your body needs. When you have an overactive thyroid, your body uses significantly more calories to carry out certain functions. In this case, your calorie surplus will have to be higher than normal.
Your protein powder will help get extra calories and protein into your diet (another important nutrient for to support weight gain). However, this doesn't mean that you can ignore the rest of your diet. You can try eating more high calorie and protein dense foods, such as red meat, whole dairy products, whole eggs, plant oils, nuts and seeds, peanut butter, avocados, etc. You can even include some "unhealthy" foods that you enjoy to help out, just don't get carried away.
Try to eat frequent meals and snacks throughout the day. With an hyperactive thyroid, it will likely be difficult to get all of the calories you need in only a couple meals. Try to snack whenever possible, even if it seems like you are force feeding sometimes.
If you incorporate your protein powder into a smoothie, you can easily increase the number of calories. Mixing it with fruit, milk, yogurt, peanut butter, ice cream, syrup, honey, etc. will greatly increase the calorie count and make for a delicious milkshake.
Keep in mind that if you have any medical conditions, you should consult with your physician before starting a new supplement. Be sure to check that medication prescribed by your doctor does not contraindicate with your personalized protein powder.