Thyroid is one of the reason for weight gain but not only reason for it. You avoid high fat,high carbohydrate,diet and take low fat low high protein, high fibre diet. Hello, do some regular physical activity of your choice to burn at least 500 calories daily to reduce 5 kg every month. Beside this cut down your calories from food by avoiding high fat,high carbs,high calorie foods and avoid carbs in dinner as well as restrict during the days also. Keep proper hydration. Sleep well to make yourself stressfree and help hormonal balance.