Difficult to say without examination. Depending on what was done, it would seem that many of the major causes have been ruled out with the blood tests. In addition to what you mentioned, I would also consider a B-12 level, blood count, metabolic panel, and aldolase/CK levels (the latter to evaluate the myositis).
If the tests continue to be non-revealing, I agree with the neurology referral. Electrodiagnostic testing can be considered to evaluate for neuropathy.
These options can be discussed with your personal physician.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin, M.D.
kevinmd_