Top Three Causes:

1) Diabetes
2) EtOH
3) Idiopathic

Consider for initial workup:
fasting serum glucose
glycosylated hemoglobin
blood urea nitrogen
creatinine
complete blood cell count
erythrocyte sedimentation rate
urinalysis
vitamin B₁₂
thyrotropin stimulating hormone levels

Always consider:
EMG
Nerve Conduction Study (NCS)
Neuro c/s if initial w/u is unrevealing