You have probably considered other medical reasons but I thought I would mention some anyway. Our body is controlled by many hormones -- not just those sexy ones
I used to get panic attacks with some very real physical symptoms. Long story short -- I had hypothyroidism. My symptom list was extensive and I had been treated individually for a handful of them -- taking all sorts of medications. Finally, my thyroid hormone pill took the place of them. Now I finally am treating the source of the problem and not each individual symptom. Hypothyroidism is not always easy to dx -- there are different types. It may have nothing to do with your panic attacks but I thought I would throw it out there. It took 4 years before the source of my problems was found and a couple more to get my meds right (I needed both T4 and T3). Just something to consider.