Because, right now, you still have thyroid hormone floating around (although no more will be made because of your total thyroidectomy)
If you began the thyroid hormone right away, (think about it) you ALREADY have too much (as you said you are hyperthyroid now, I am not familiar withe 3+ number, I am only familiar with lab values…..) so why add more to it?
You are still pretty post op, and you will be tired, primarily because that is how you feel after surgery and anesthesia.
My own experience is that after my surgery, it was about a month or so before I began my Synthroid. Depends on labs and how you feel.
Shirley