I read of the possible links (at least in some cases) between bipolar disorder and thyroid function several years ago. According to an old book "Surviving Manic Depression" by Torrey, it has been looked at for quite a while.

I read that before any firm bipolar diagnosis, a person should ideally have a thyroid function test, maybe others, a physical, and be asked many questions about any substances (including OTC stuff) that are/were taken. These in addition to the usual questions.

I had all of the above during my first hospitalization, but had received a bipolar diagnosis a couple years before. My thyroid function was normal at the time. I only developed hypothyroidism after taking Lithium for a while, though I had a grandmother who had thyroid issues without bipolar meds. She like almost 50% of middle-aged to elderly people develop thyroid issues, though not necessarily Hashimotos, whose stats are more 1-2% of population, with more women than men.

I would definitely not jump to any conclusions that a bipolar diagnosis is inaccurate due to thyroid issues!

I also personally know of a couple people treated with thyroid meds for their bipolar, without clear thyroid issues. I think as referenced in the article, it's commonly used in some people with rapid (or more ultra rapid) cycling. At least the two people I reference said that's why theirs was prescribed.

I take 150 mcg Synthroid/Levothyroxine and have taken it for at least 11 or 12 years. It doesn't stop my episodes, nor does it obviously prevent them (or that I know). I am not an ultra rapid cycler generally. Whether my Synthroid makes any difference in that, I don't know. Generally the only time I ever experienced ultra rapid cycling was when I was on an antidepressant, with or without a moodstabilizer and antipsychotic.