SIADH as an inappropriately low osmostat

SIADH is the exact mirror of diabetes insipidus. A sustained inappropriate antidiuresis — modeled as a downward shift of the defended osmolality — holds osmolality below setpoint (a hyponatremia surrogate), reaching 272 mOsm/kg while DI rises to 309 around the 287 setpoint. Grade [V]; SIADH Na<135 anchor [L]; volume escape [O].

SIADH applies a sustained antidiuretic bias to the same osmostat, represented as a downward shift of the defended osmolality. Osmolality settles below the 287 setpoint, deeper with larger shift (284→272 mOsm/kg), reaching the hyponatremia band — the sign-flipped image of the DI run on one loop.

Antidiuresis that defends the wrong setpoint

In SIADH antidiuretic hormone is secreted inappropriately, so water is retained even when plasma is already dilute, producing euvolemic hyponatremia (serum Na<135; Verbalis JG et al., 2013). On the osmostat the cleanest minimal representation is a downward shift of the defended osmolality: the loop now stably holds a low osmolality.

The mirror image of DI

Shifting the defended osmolality down 284→272 mOsm/kg, the loop settles osmolality below setpoint and crosses the hyponatremia band (≤275, a Na<135 surrogate). Placed beside the previous section this is an exact mirror on one mechanism: diabetes insipidus drives osmolality above the 287 setpoint (to 309), SIADH holds it below (to 272). The direction and the DI/SIADH symmetry are reproduced [V]; the minimal osmostat omits the volume-mediated ADH escape that caps real SIADH, so absolute sodium and the escape kinetics remain [O].