I assume that you are on thyroid hormone replacement
ever since your surgery. If not, then you should immediately start on Tab Thyroxine
100mcg once daily. The dose will have to be titrated over a few months by getting monthly serum TSH
done till it is normal or low. If you are on this medication then your belly fat is less likely to be due to your disease and more due to your diet and habits. However, just to rule out an organic cause, you should get a serum TSH done. If it is normal then you have nothing to worry about. Otherwise you will need to change the dose. If it is high you will have to decrease the dose and vice-versa. Your disease or surgery has no effect on your immunity and i have the same immunity advice for you as the rest--take healthy vitamin rich food, less oil and sugars, excercise daily.