To my understanding, it's POTS as along as it causes the increase in heart rate upon standing (forget the amounts, I think increase in 30 or to above 120bpm?). So your symptoms may be caused by thyroid issues, but I would still think that they are POTS. I haven't tried any meds yet as I'm super sensitive to any drugs, but there are many lifestyle changes to try first. Have you tried any, or did your doctor put you straight on the beta blockers? Let us know if you don't know much about possible lifestyle changes, as there are many threads and links with lots of help.
Have you had any other menopause-like symptoms to fit that theory? I would assume that the normal thyroid results (because you're on correct thyroid meds dose?) would indicate that your hypothyroidism is under control and thus not causing the POTS. Are your normal results include TSH, T3 and T4? It can be hard to know definitively, but your normal results mean your body should have enough circulating hormones to function normally (for the most part!).
Also do you have any other autonomic symptoms that started around when the POTS type symptoms started? For example, I started getting an exaggerated startle response (overwhelming shock from a small surprise, had to rest for a bit after each time), thermoregulation issues, sweating at inappropriate times and all over (not focused at normal areas like armpits), palpitations, night sweats, GI issues.
I think it can be really hard to work out what is the cause and what is the result when dealing with the many syndromes and symptoms we all seem to suffer from. I think your best bet will be to do what you can to address each issue - I list my strategies for each issue separately, then go through my lists and ensure that all strategies work ok with each other (ie. you don't want to try a POTS treatment if it might mess up your thyroid). In my experience, doctors VERY easily miss contraindications for complicated patients. Having lists of my strategies helps remind me to stick with them too, otherwise there's too much to remember!