[Disclaimer: I am 100% not trying to invalidate anyone else's diagnosis. It just never occurred to me that my "BPD" might actually have a biological basis, and I don't know if my story could help someone else.]

I was initially diagnosed with BPD due to 2 years of emotional instability, feeling empty and suicidal, acting impulsively, etc. I was on a waiting list for counselling for about 2 years, and then received some 1-2-1 sessions and group mindfullness. The mindfullness and an exercise routine helped me regulate my moods somewhat. The therapist was less helpful. She seemed stumped that I hadn't experienced any trauma as a child, so suggested the BPD had been caused by an unstable childhood because I'd moved twice. This didn't ring very true to me, as I'd always been surrounded by loving stable family members.

During this time I also saw multiple doctors about menstrual problems. I was bleeding pretty much constantly for 5 years and suffered from horrendous cramps. I had a laparoscopy to rule out endometriosis and PCOS and tried 4 different types of birth control.

After 5 years my gynaecologist finally figured out that all the birth control I'd been on were progesterone-based and switched me to a copper coil. All the symptoms started clearing up almost immediately - including the emotional ones. I have a progesterone intolerance that causes mood swings, not BPD. It just took far too long for a doctor to realise that my menstrual and mental issues could have a common cause - hormones. 

It worries me that not a single doctor in 5 years thought of ruling out a biological cause for emotional instability, instead prescribing anti-psychotics and referrals to therapists.