Some of the symptoms sound like they could possibly be dysautonomia, but these are nonspecific symptoms that could also be the symptoms of any number of other things, and you have quite a bit beyond autonomic symptoms going on such as your thyroid disorder, hepatitis, and your "stiff heart."
Before I address anything else, I want to talk about your doctor's discussion regarding your heart being stiffer than other people's. This may be a matter of degree of severity in terms of why he is telling you not to worry, as in perhaps in your case the stiffness is not yet very severe?? Let me preface this with the disclaimer: I am not a doctor and I am only attempting to explain this as I understand it as a fellow patient. Take this with a hefty grain of salt. That being said ...
I'm going to pull here from Stoelting, Robert K. & Ronald D. Miller. Basics of Anesthesia. Philadelphia: Churchill Livingstone, 2007. pp. 49-50.
Okay, so you have what is known as your pulse pressure, which is your Systolic Blood Pressure--the top number--minus your Diastolic Blood Pressure--the bottom number. "The pulse pressure is created by the addition of stroke volume"--the volume of blood pumped from one ventricle of the heart with each beat--"on top of a [diastolic blood pressure] within the compliant vascular tree." Therein lies the rub--"compliant." It goes on to point out that if there is "poor aortic compliance" (in other words, stiffness), increased pulse pressure can occur.
So returning to your case in particular, looking at the one specific example of blood pressure you give above: 175/91. We get a pulse pressure of 175-91 or 84. A normal resting seated pulse pressure for a healthy adult is somewhere around 40. A pulse pressure as high as 84 could be normal and reflective of high stroke volume if you were exercising at the time, but if you're resting, it's likely a reflection of poor compliance or "stiffness" as your doctor put it.
Now here's where I don my "I'm not a doctor" hat. Everything I can find anywhere online is saying that this is associated with cardiovascular complications and congestive heart failure. Maybe your pulse pressure isn't ordinarily that high. Maybe your particular situation isn't very advanced. Maybe I'm reading this all wrong. But if I were you I would get back with this cardiologist (or get a second opinion) immediately to get to the bottom of this; there's no reason a doctor should be so vague about something as serious as the condition of your heart.
Moving on, you may already have the feeling you've hit the end of your rope with endocrinologists. You definitely need to keep up with one for your thyroid issues (and if you haven't found one you're happy with, move on and try a fourth--you wouldn't be the first person to have to go through doctor after doctor until you settled on one with whom you clicked). But you may have to branch out to get assessed for other possible problems. Do you have a good general practitioner that is heading up your case? Has he/she suggested referrals to other specialties or considered other tests? If you're hitting a wall on that front, *that* may be the source of your problem. A good GP should have a "plan B" and a "plan C," etc. when you come back from one specialist or another without answers. Someone suggested autoimmune; if basic autoimmune markers haven't been tested yet, those are easy blood tests that definitely should be run to screen for that angle.
As for dysautonomia-specific testing, you have done the cardiologist route, so maybe you already have some test results that would give us more insight. Have you had a holter monitor? Echocardiogram? Stress echo? Any other cardiology testing (EP study, etc.)? Do you have specific results from any of these tests (the more specific the better)? If there are indications that point to dysautonomia, a tilt table test could be requested, but I think it's unlikely you'll get this until they see some indication from either a holter or ambulatory blood pressure monitoring that the tilt is indicated, unless you have syncope (which you haven't mentioned).
Here is more information on diagnosing dysautonomia:
http://www.medhelp.org/health_pages/Neurological-Disorders/Diagnosing-Dysautonomia/show/827?cid=196
On finding dysautonomia specialists (though generally one would seek them out after there's a little more evidence established as I said through holter monitoring or other tests from local docs):
http://www.medhelp.org/health_pages/Neurological-Disorders/Dysautonomia-Specialists/show/717?cid=196
Are the doctors you're seeing at a major teaching hospital/what hospital system are you using? Are you willing to travel to access a higher-ranked hospital system, and if so, how far are you willing/able to travel?
Sorry to ask so many questions, and sorry this is such a long post. I really hope you don't feel bombarded, I'm just trying to cover all the bases here. Feel free to respond in bits and pieces. In the meantime, if you're looking for more reading material on dysautonomia or the various forms thereof, we keep a brief reading list here:
http://www.medhelp.org/health_pages/Neurological-Disorders/Further-Reading-on-Dysautonomia/show/696?cid=196
I hope I've addressed most of what you've asked. Feel free to ask more and I'll try to keep up. Bear with me. Thanks.