Thanks for the reply. I have had my thyroid function tested a few times over the past few years. I've had a test for diabetes, Addison's disease, gut inflammation, calprotectin, kidney function, h pylori. I've also had a head CT scan, mrcp and gastroparesis breath test, and a pilcam which showed healing ulcers and lymphangiectasia, though my consultant assured me that this is a common finding, and won't be causing my symptoms as it's just a single one.
I also had a heart rate variability test which showed I have very little parasympathetic activity.