Yes, I've heard of the thyroid/bipolar connection before -- I have hypothyroidism myself. The thyroid problems mimic depression, so often times people who are depressed will be treated as such and make no progress in their treatment because they're actually having problems with their thyroid. This is very common among bipolars. I'm not sure why. I should ask my doc someday.
Generally, I think the best way to frame the conversation is to approach her with the "I'm worried about you because..." and "I see this... happening and it worries me because ..." The hardest part is to convince someone to go to the doctor. One way to do that is to offer to go with her. If she seems very tense about it, but close to agreeing, offer to take her. That might help.
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