A patient with SIADH has serum Na⁺ of 118 mEq/L, plasma osmolality 245 mOsm/kg, and urine osmolality 620 mOsm/kg. The appropriate physiological response to hypo-osmolality — suppression of ADH and production of maximally dilute urine — is absent because:
- A Renal tubules develop resistance to ADH
- B Aldosterone excess overrides the osmoreceptor signal
- C Hypothalamic osmoreceptors are reset to a lower threshold in SIADH, but ADH secretion continues inappropriately independent of osmolality ✓
- D Baroreceptor-mediated ADH release overrides osmotic suppression due to severe volume depletion
Correct answer: C. Hypothalamic osmoreceptors are reset to a lower threshold in SIADH, but ADH secretion continues inappropriately independent of osmolality
Explanation
In SIADH, ADH is secreted at levels that are inappropriately high for the prevailing low plasma osmolality. The hallmark is that the normal osmoreceptor-driven suppression of ADH does not occur; ADH may be secreted ectopically (e.g., small-cell lung carcinoma) or from a non-osmotically stimulated posterior pituitary. The result is persistent AQP2 insertion and concentrated urine despite plasma hypo-osmolality. This is distinguished from volume-depletion states where baroreceptor-driven ADH release is physiologically appropriate.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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