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Editor, – I read with interest the addition of tramadol to the already long list of medications that cause the syndrome of inappropriate antidiuretic hormone secretion (SIADH) (Aust Prescr 2004;27:97). The temporal relationship between serum sodium level and tramadol use appears to have secured the diagnosis.

While not so relevant to the elderly population, SIADH is essentially a diagnosis of exclusion where, in the presence of normovolaemia, other sinister (if not treatable) causes have been excluded.1 It is important that readers should not get the impression that the diagnosis is based on solitary serum sodium and osmolality measurements. It is critical that, together with other osmotically active analytes, urinary sodium and urinary osmolality are measured in parallel in the overall assessment of hyponatraemia. This will assist in further understanding the pathophysiology which remains, as stated, hypothetical at this stage.

Huy A. Tran
Head, Department of Clinical Chemistry
Hunter Area Pathology Service
John Hunter Hospital
Newcastle, NSW


  1. Robinson AG, Verbalis JG. Posterior pituitary gland. In: Larsen PR, Kronenberg HM, Melmed S, Polonsky KS, editors. Williams textbook of endocrinology. 10th ed. Philadelphia: W.B. Saunders; 2003. p. 281-329.