This is imprtant to understand that your thyroidectomy
is not an emergency surgery to treat some life threating condition. It is an elective surgical procedure. Before any planned surgical procedure it iimportant to make sure that you are completly fit for the surgery and ready to face the anaesthesia.
Now based on the assesement of your compelte check up only it can be decided to go for the surgery.
So don not got mad on doctor, he/she is doing this for the safety of yours and once every thing is within the normal limits for the surgery, it can be done.
As far as the boderline enlarge heart is concern, it is important to take that in your condition because of thyroid problem.
Thanks and take care