How is the diagnosis of SIADH?

There is no single lab test confirmatory of it. Hyponatremia with hypoosmolality, continued renal excretion of sodium, urine less than maximally dilute, no volume depletion, absence of other causes of hyponatremia (adrenal insuffiency or hypothyroidism) and correction of hyponatremia by fluid restriction (It is a dx of elimination- tested by dehydration)