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
I think its an orthapeadic doctor
The Dermatologist (skin doctor)
Foot doctor
an oncologist
A psychologist.
gastroenteroligist
nuerologist
A gastroenterologist treats stomach pain.
A GP should do.
animal doc.
podiatrist