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Difficulty: HardEndocrinology and Metabolism

Match each of the following clinical vignettes and laboratory profiles to the most accurate underlying pathophysiologic mechanism.

  • A 2828-year-old with bipolar disorder presenting with polyuria. Laboratory evaluation shows serum Na+Na^+ 148148 mEq/L and urine osmolality 120120 mOsm/kg. Exogenous desmopressin administration produces no significant change in urine osmolality.Renal collecting duct resistance to antidiuretic hormone
  • A 4545-year-old recovering from transsphenoidal adenomectomy develops excessive thirst. Laboratory evaluation shows serum Na+Na^+ 149149 mEq/L and urine osmolality 140140 mOsm/kg. Desmopressin administration increases urine osmolality to 580580 mOsm/kg.Deficient posterior pituitary release of antidiuretic hormone
  • A 6565-year-old with a chronic cough and weight loss presents with confusion. Laboratory evaluation shows serum Na+Na^+ 122122 mEq/L, urine osmolality 550550 mOsm/kg, and urine Na+Na^+ 4545 mEq/L.Unregulated, non-physiologic excess of antidiuretic hormone
  • A 3434-year-old with schizophrenia reports drinking 88 liters of water daily. Laboratory evaluation shows serum Na+Na^+ 131131 mEq/L and urine osmolality 8585 mOsm/kg. A supervised water deprivation test gradually increases urine osmolality to 600600 mOsm/kg.Appropriate physiologic suppression of antidiuretic hormone release

Answer

The correct matches pair the lithium patient with renal resistance (nephrogenic DI), the post-surgical patient with deficient pituitary release (central DI), the patient with suspected lung malignancy with unregulated excess ADH (SIADH), and the compulsive water drinker with physiologic ADH suppression (primary polydipsia).
Matching these conditions requires synthesizing baseline sodium, baseline urine concentration, and dynamic testing results. Hypernatremic patients with dilute urine have diabetes insipidus (DI); a history of lithium therapy combined with a failure to respond to desmopressin confirms nephrogenic DI (renal resistance), whereas a history of pituitary surgery and a robust response to desmopressin confirms central DI (deficient release). Hyponatremic patients with inappropriately concentrated urine have SIADH (unregulated excess ADH), frequently driven by ectopic production from a malignancy. Hyponatremic patients with appropriately dilute urine who subsequently concentrate their urine during water deprivation have primary polydipsia (appropriate physiologic suppression of ADH).

Step-by-Step Solution

1
Analyze baseline serum sodium to determine overall body water status.
Hypernatremia (>145>145 mEq/L) indicates a primary loss of free water (diabetes insipidus), whereas hyponatremia (<135<135 mEq/L) indicates primary water retention (SIADH or primary polydipsia).
This fundamental step categorizes the patient into a water-losing versus water-retaining state.
2
Evaluate baseline urine osmolality in the context of the serum sodium.
Low urine osmolality (<300<300 mOsm/kg) despite hypernatremia confirms a severe concentrating defect. High urine osmolality (>100>100 mOsm/kg) despite hyponatremia confirms an inability to dilute the urine.
Urine osmolality demonstrates whether the kidneys are responding appropriately to the abnormal serum tonicity.
3
Interpret the response to dynamic testing (desmopressin administration or water deprivation).
A response to desmopressin differentiates central DI (responds) from nephrogenic DI (does not respond). A normal response to water deprivation differentiates primary polydipsia (concentrates normally once water is withheld) from DI.
Dynamic testing isolates the specific anatomical failure point (brain versus kidney) or behavioral cause in the antidiuretic hormone axis.

Key Concept

Diagnosis of Polyuria and Sodium Derangements via the ADH Axis
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Topics

Endocrinology and Metabolism

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