I actually am being treated for an underactive thyroid but that's because my thyroid was killed off after being hyper! But as a side note, CD doesn't seem to have any effect on the hypo and medication side effects which is what I was worried about. Incase you happen to think of it!
Another thing to keep in mind, I'm assuming you're taking the same type of medication as I am and with the replacement meds, you cant take them within 4 hours of taking any vitamin. I take the replacement at night and just make sure I don't drink a CD shake after 6pm-ish since it does have a decent dose of calcium.
Perhaps I'm being too careful but I'd rather be more careful with it.