Hello Dr, three years ago I had a total thyroidectomy, ever since I take Oroxine 100 mg nil on Sundays why do I feel dizzy and lightheaded on Sundays?

Difficult to say. Not taking the Oroxine 1x weekly should not decrease your thyroid hormone levels significantly enough to cause symptoms, since the half life is about 7 days. Talk to your Dr. and see if you can take it on Sunday as well to see if it fixes your symptoms.