Why do we use the term SIAD instead of SIADH?

Why do we use the term SIAD instead of SIADH?

• A. Not all patients show elevated circulating levels of ADH
• B. All patients have elevated ADH
• C. The name originated from a different hormone
• D. It describes a different syndrome altogether

Answer: (A)

The main point is that the condition is defined by inappropriate antidiuresis—the kidneys reabsorbing too much water—rather than by consistently elevated circulating ADH levels. Measuring ADH is tricky: it has a short half-life and is not routinely detectable in every patient who has hyponatremia due to inappropriate water retention. As a result, some patients with hyponatremia and concentrated urine may have normal or undetectable ADH levels at the time of testing, yet the syndrome persists because the antidiuretic effect is still driving water reabsorption. That’s why the terminology shifted to SIAD (syndrome of inappropriate antidiuresis) to reflect the functional problem rather than assuming a constant rise in ADH.

The main point is that the condition is defined by inappropriate antidiuresis—the kidneys reabsorbing too much water—rather than by consistently elevated circulating ADH levels. Measuring ADH is tricky: it has a short half-life and is not routinely detectable in every patient who has hyponatremia due to inappropriate water retention. As a result, some patients with hyponatremia and concentrated urine may have normal or undetectable ADH levels at the time of testing, yet the syndrome persists because the antidiuretic effect is still driving water reabsorption. That’s why the terminology shifted to SIAD (syndrome of inappropriate antidiuresis) to reflect the functional problem rather than assuming a constant rise in ADH.