Hyponatremia with Isovolemia is seen in-
The syndrome of inappropriate secretion of antidiuretic hormone (SIADH) is a disorder of impaired water excretion caused by the inability to suppress the secretion of antidiuretic hormone (ADH) . If water intake exceeds the reduced urine output, the ensuing water retention leads to the development of hyponatremia Ref Davidson 23rd edition pg 366
SIADH
Conn's syndrome
Cushing' syndrome
Frusemide therapy
Fluid and electrolytes