Foods with a very high Glycemic index aren't really good for anyone. These foods are usually very processed (such as store bought white bread), can be high in fake sugars (high fructose corn syrup), etc. (Basically low in everything but sugar and starch.) They are all processed very quickly, so that can result in huge variations in blood sugar levels. A life time with lots of high GI foods can make you more prone to diabetes and heart disease. So it may also have a huge effect on your weight and cholesterol.
Personally I was never told to worry about high GI foods from my dr for any other reason besides a family history of heart disease and diabetes. However, if you're like me, my thyroid has done a lot of damage to my body. I'm 55lbs over weight and have high cholesterol and blood pressure- the last thing I need is a bad diet. I don't know if it will specifically react to my med- levothyroxine. BUT it's not good for me anyways because I'm already at risk due to family history and more at risk due to my own body. Perhaps this is the reason why you were told to avoid them.
I was never told this either. However I try to avoid sugar, white breads ,pastas, and all cereals other than oatmeal and All Bran. It's a good way to keep weight and mood swings in check and avoid the free radical damage caused by sugar.
Why exactly were you told to avoid high glycemic foods w/ levo.? Is there an interaction w/ the drug?