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What is Reset Osmostat?

Reset osmostat is a subtype of SIADH (type C in the nephrology literature). It is essentially your body having a higher threshold for low sodium. In other words your body will allow the sodium to run lower, to a lower set point than what is typical1. As you drink water, under normal circumstances, the body will suppress ADH and your kidneys will release that water, keeping blood sodium levels in a narrow range. In SIADH this does not occur and high water intake dilutes the sodium. In reset osmostat only as the sodium tracks closer to normal does that secretion of ADH take hold. Due to this those with reset osmostat tend to stay consistently in the 125-135 mEq/L serum sodium range. This is a new normal for that individual patient, and the system is still intact just at a lower set point. This phenomenon is actually a normal occurrence in pregnancy. Some clinical clues to the diagnoses of Reset Osmostat include1,2:

•Sodium in the 125-134 range for several days despite varying water and salt intake
•Fractional Excretion of Uric Acid in the normal 4-11% range
•Rule out primary polydypsia

​•Greater than 80% of a water load (20 ml/kg) excreted in a 4 hour period (low urine osm)

The idea is this is asymptomatic and no definitive treatment is absolutely indicated. However most clinicians would still try to correct the sodium based on the fact that even mild Hyponatremia is correlated with significant morbidity and mortality.

​1. Indian J Nephrol. 2019 Jul-Aug; 29(4): 232–234.

2. Ten common pitfalls in the evaluation of patients with hyponatremia Filippatos, T.D. et al.European Journal of Internal Medicine, Volume 29, 22 – 25