An interesting question came up recently in a Hyponatremia forum regarding why someone with SIADH specific gravity (concentration of urine) would be so varied. If I can still concentrate and dilute my urine why then do I still suffer from Hyponatremia, she asked. While the answer, of course, is far from simple one obvious conclusion is well you probably have type A.
There are 4 types of SIADH.
1. Variable and erratic vasopressin release.
2. Low level basal continuous secretion.
3. A reset osmostat type which responds normally just at a lower set point.
4. The so called Nephrogenic SIADH where a gain of function mutation at the V2 receptor leads to a constant ADH like effect. This has low levels of vasopressin.
The most obvious next question is does this change the management? I would say absolutely yes. The typical erratic SIADH type A and constant type B would require the usual approach. This is fluid restriction, high protein diet, UreaAide, Vaptan and so forth. If you however have the reset osmostat, type C, you may have more difficulty in achieving a normal sodium level, and perhaps not as imperative we do so. Lastly type D, for example will be completely unresponsive to vaptan therapies.
In closing most folks with SIADH likely fall into type A or B SIADH, but reset osmostat maybe more challenging to treat chronically and type D although rare, will not respond to typical therapies. Thus concluding yes type does matter.
1. Hyponatremia secondary to the SIADH: Theraputic decision-making in real-life cases. October 2013 CKJ.
2. UpToDate SIADH diagnosis and treatment. Accessed July 5, 2021.
3. Credit Hyponatremia support group Facebook group for the great question.