Just about any medical doctor can treat low sodium but the one most likely would be one who has specialized in internal medicine.
Dermatologist treats that condition
The Dermatologist (skin doctor)
Foot doctor
I think its an orthapeadic doctor
an oncologist
nuerologist
gastroenteroligist
A psychologist.
A gastroenterologist treats stomach pain.
A GP should do.
animal doc.
podiatrist