A generalist, internist, or an endocrinologist. You may be referred to a nephrologist.
Dermatologist treats that condition
The Dermatologist (skin doctor)
Foot doctor
I think its an orthapeadic doctor
an oncologist
A psychologist.
gastroenteroligist
nuerologist
A gastroenterologist treats stomach pain.
A GP should do.
If you have hyponatremia, start with your primary care provider. The initial testing that your PCP can do will direct you appropriately to the right specialist. There is no one specialist who treats hyponatremia; instead, you have to narrow the possible causes.
Pediatrician.