Please don't start any medication which can alter your hormone levels. Thyroid hormones play key role in all of your body functions. And in case of Hyperthyroidism, without blood investigations you should not go ahead with medicine.

I can tell you the best about diet and gaining weight. Before that I need to calculate ur BMI? How much is your weight and height? How much you do on an average day? These all things will help me to calculate your intake calories and accordingly I will tell you your diet plan

You can have a multivitamin without any check up but i strongly oppose taking any thyroid medication without undergoing a blood check up for it, you may require other blood investigations as well to find the underlying cause for the loss of weight and muscle. These following steps may help you with gaining weight and muscle: - Eat healthy, avoid junk food, Increase your calorie intake, eat more calories than your body burns in order to gain weight. Aim for 300-500 calories per day above your maintenance level - Eat increased amount of protein as protein will help in muscle gain - In order to gain weight, eat at least 3 meals per day and make sure to eat plenty of fat, carbs and protein in each meal. - Eat food like nuts, dried fruit, high-fat dairy, Fats and Oils, grains, meat, tubers, dark chocolate, avocados, peanut butter, coconut milk, granola, trail mixes. - Drink plenty of fluids(3 liters per day) - Exercise regularly, you can join the gym to gain muscle as well I would advise you to do the whole body check up to rule out any underlying cause for the loss of weight