Re: soy and synthroid
I don't know if soy also interacts with Synthroid, I've never seen anything that has said that. However, almost anyone who knows what they're talking about will say not to consume soy. Soy is a goitrogens, so if consumed often will help to create a goiter.
I wouldn't suggest it. Google soy and hypothyroidism, there's a lot of information about it.
It might mean having to drink coffee without milk. But caffeine isn't very good for your thyroid either, so it might just be best to stop drinking it all together.