The reason the doctor asked you to lower the thyroid medication is that Niacin will after time affect your T3 T4 levels. Even without any symptoms. So best to do as the doctor says. I presume you have high cholesterol and that is why you are on the Niacin?
Did you get the actual results or did they just tell you "normal"? if not, always get your own copy.....
I am asking, because if the levels are ok, then I don't understand why the doc would adjust. them