What are you taking for your thyroid issue?
Since how many days have you noticed weight gain.
Have a high protein, low carbohydrate low fat and high fiber diet.
Let your meals consist mostly of vegetables and fruits.
Avoid foods with high glycemic index like rice, coconut etc.
Take up cardio excercises like running, jumping jacks, playing certain sports like badminton etc as these excercises will keep your heart rate in the range of 140-150 which is most beneficial for weight loss.
You should also really concentrate on your calorie intake.
You should take in less than required and concentrate on burning more than what you have taken.
Consult a dietician for a detailed plan if you don't understand.