I think the key is to put away all the anger and resentment (she senses it even when you don't verbalize it, trust me) and approach her in the most loving way possible. Just tell her that you're really concerned, you know that fatigue is one of the symptoms of the thyroid disease, and ask her when is she due to have her blood levels checked again?
Trust me, she will be like a new woman when you get this all straightened out. It happens so gradually, though, that right now she's like a boiled frog - she doesn't even realize how bad it's gotten.
