Does soy based food have to be completly avoided if i'm taking levothyroxine?

Not at all: There is no problem or interaction whatsoever. The brand name thyroxine should be taken fasting, but actually can be taken any time as long as you're consistent with administration and have your levels checked to be normal. Can't take it within two hours of vitamins, iron or calcium however.

Soy and thyroid: If you have thyroid tissue, and require levothyroxine because your existing thyroid is underactive, then soy intake as a regular food can make your thyroid tissue less active; it inhibits iodine metabolism; if you had your thyroid removed surgically, then soy will not interfere with levothyroxine.