Isotonic saline is NOT given for SIADH, as this can worsen the effects of hyponatremia. Per UpToDate (section within treatment of hyponatremia of SIADH), isotonic saline should not be given as "Assuming the patient is euvolemic, the administered sodium is excreted in the urine because the response to aldosterone and atrial natriuretic peptide is normal. However, the water is retained because of the persistent action of ADH. Thus, when 1 liter of isotonic saline is administered to a patient with SIADH, the sodium is excreted in the urine while some of the water is retained, worsening the hyponatremia.".
This has resulted in fatalities and has been shown in a number of clinical studies.
Uptodate article name "Overview of the treatment of hyponatremia in adults" -> section ("approaches that we typically avoid -> Do not use isotonic saline in SIADH")
Resubmit and cut back on info in extra and we'll reconsider this, clearing queue of some old suggestions