Patient Care

How Is SIADH Diagnosed and Managed?

As is common in clinical practice, her true volume status is difficult to clinically ascertain. By physical exam, she appears euvolemic, but because she is on hydrochlorothiazide, she might be subtly hypovolemic. The UNa of 40 mmol/L is not consistent with hypovolemia, but its accuracy is limited in the setting of diuretics. The failure to improve her sodium by at least 5 mmol/L after a 2 L normal saline infusion argues against low effective arterial blood volume and indicates that the hydrochlorothiazide is unlikely to be the cause of her hyponatremia.

Therefore, the most likely cause of the hyponatremia is SIADH, a diagnosis further corroborated by the elevated FEUrate of 13%. Her chronic hyponatremia should be managed initially with fluid restriction while an investigation for an underlying cause of SIADH is initiated.

Bottom Line

The diagnosis of SIADH relies on the careful evaluation of laboratory values, use of an algorithm, and recognizing the limitations of clinically assessing volume status. The underlying cause of SIADH must also be sought and treated. TH

Dr. Grant is a clinical lecturer in internal medicine, Dr. Cho is a clinical instructor in internal medicine, and Dr. Nichani is an assistant professor of internal medicine at the University of Michigan Hospital and Health Systems in Ann Arbor.

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