Since you appear to have a formal diagnosis (??), I would assume that you are on a monitored rate of Synthroid or some other suitable remedy?

Your HR should....SHOULD...rise to a utile level based on your level of exertion and oxygen debt/CO2 levels (hypercapnia). If you are precluded from any useful exertion, say wheeling some sod away from a place you are clearing in your yard, then you may need further assessment, and possibly a modest rise in your prescribed thyroxin substitute. That's just a guess...I'm not an endocrinologist.

I don't know anything about adrenal insufficiency. I have heard of overactive adrenal glands, which is a whole 'nuther issue.

Finally, I happen to think, and it's just me based on a lot of reading and listening to cardiologists over the seven years I have had atrial fibrillation (AF), but a resting HR of 45 is NOT a problem. It just isn't. Down to 30-35, yes, and get help soon...but 45 for an otherwise healthy and resting heart is nothing to be concerned about. It's when you want to impose a rising demand on your heart, and cannot...there is where you should actively pursue the matter and get it dealt with. My opinion, strictly.