A 55-year-old man with small cell lung carcinoma develops serum sodium of 122 mEq/L with urine osmolality of 450 mOsm/kg and urine sodium of 55 mEq/L. His volume status is euvolemic. ADH levels are elevated. The ectopic ADH production by tumor cells differs from physiological ADH secretion in which important respect?

• A Tumor ADH lacks antidiuretic activity at the V2 receptor
• B Tumor ADH acts only on V1 receptors, not V2 receptors
• C Tumor ADH cannot be measured by standard immunoassays
• D Tumor ADH secretion is not regulated by plasma osmolality ✓

Explanation

In SIADH due to ectopic ADH production (classically small cell lung cancer), ADH secretion is autonomous and unregulated by normal physiological controls such as plasma osmolality or volume status. The ADH is biologically active at V2 receptors, causing water retention and concentrated urine. It is detectable by immunoassay. The key distinction is the loss of osmotic regulation.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.