As one who has the hypothyroid issures [now autoimmune], autoimmune neuropathies and have had breast cancer. I can attest that there is NO way the first two are impirically connected to the last one. The docs didn't know to check, even w/a very strong family history of cancers. I suppose that justifying the specialized tests for insurance are also an issue, but I'm not gonna go down that maze for see if there are some should-haves. It's done.
The numbers of folks who have all three issues is small. That the cancer could have precipitated the other issues the odds are minutely smaller indeed. I know only about 4-6 folks who have any combo of these issues on any boards. Only one person clearly had paraneoplastic neuropathy.
Do not worry about it until THE IF AND WHEN of anything happening. IF anything happens, which likely it shouldn't.
That your skin biopsy was negative is GOOD news! That you are getting other specialized testing such as the autonomic ones is well, well above average of investigations by any doctor or doctors. It sure sounds as if your docs are trying hard to get to the root of your problem! That the biopsy was NEG? Well, that's one of many things that's to be checked off the diagnostic list! That test list is usually very long, so keep at it.
So think positively as best you can and keep plugging at it. You might find out that it's something simpler than first thought. I sure hope so!
Hugs and hope always 's - j