Many medications: commonly: thiazides and thiazide-like diuretics, SSRI's (esp citalopram), anti-psychotics and many, many others. Hyponatraemia does not always start on commencement of that med.
respiratory pathology: pneumonia, active TB, abscess, neoplasm, or asthma
CNS pathology: infection, trauma, or neoplasm.
Various carcinomas: nasopharynx, duodenum, stomach, pancreas, ureter, prostate, or uterus
hypothyroidism, adrenal insufficiency
Sodium losses: diarrhoea and sweating
In setting of hypervolaemia:
Urinary Na+ >20
Urinary Na+ <20
Nephrotic syndrome
Cirrhosis
CHF