I have had Hashimoto's (an underactive thyroid) for almost 20 years. I take my thyroxine and that's pretty much it. However I do a deep internet dive every now and then and it seems to be an area where many doctors are highly dismissive. It's my perception that there's a lot of controversy out there about how seriously to take and treat thyroid conditions. So I'm not surprised by the reaction you've had from your doctor.
For the record, my BC diagnosis and treatment has not changed my thyroid function. Good luck, K xox