Endocrine · Year 2 · from Endocrine

Case 2: Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH)

Patient Demographics

  • Age: 68 years
  • Sex: Male
  • Occupation: Retired postal worker

Chief Complaint

"I've been confused and weak for the past few days."

History of Present Illness

A 68-year-old man with a 50 pack-year smoking history is brought to the emergency department by his daughter due to progressive confusion and weakness over 4 days. He has had decreased appetite, nausea, and mild headache. He has not had vomiting, fever, or focal neurologic symptoms. His daughter notes he seems "not himself" and has been unsteady when walking. He has no history of liver, kidney, or heart disease. He has lost 15 pounds unintentionally over the past 3 months and has developed a new cough.

Physical Examination

  • Vital Signs: BP 138/82 mmHg, HR 76 bpm, Temp 37.1°C
  • General: Confused, oriented to person and place but not time
  • HEENT: Moist mucous membranes, no JVD
  • Cardiovascular: Normal heart sounds, no peripheral edema
  • Pulmonary: Decreased breath sounds right upper lobe
  • Neurologic: Slow to answer questions, mild ataxia, no focal deficits
  • Volume status: Clinically euvolemic

Workup

  • Laboratory Studies:
  • Serum sodium: 118 mEq/L (severely low)
  • Serum osmolality: 248 mOsm/kg (low)
  • Urine osmolality: 520 mOsm/kg (inappropriately concentrated)
  • Urine sodium: 68 mEq/L (elevated >40)
  • BUN: 8 mg/dL (low)
  • Creatinine: 0.7 mg/dL
  • Uric acid: 2.8 mg/dL (low)
  • TSH: 2.4 mIU/L (normal)
  • Morning cortisol: 14 μg/dL (normal)
  • Chest X-ray: 3 cm right upper lobe mass
  • CT Chest: Right upper lobe mass concerning for malignancy with mediastinal lymphadenopathy

Diagnosis

SIADH secondary to suspected small cell lung carcinoma (paraneoplastic)

Treatment

  1. Immediate management:
  • Fluid restriction (800-1000 mL/day)
  • Hypertonic saline (3%) if severe symptoms (seizures, severe confusion)
  • Target sodium correction: 6-8 mEq/L in first 24 hours (avoid >10-12 mEq/L/day to prevent osmotic demyelination syndrome)
  1. If refractory to fluid restriction:
  • Salt tablets with loop diuretic
  • Vasopressin receptor antagonists (tolvaptan) - use with caution
  • Demeclocycline (induces nephrogenic DI)
  1. Treat underlying cause: Oncology referral for lung cancer evaluation and treatment
  2. Monitor: Serum sodium every 4-6 hours during acute correction

Clinical Pearl

SIADH is characterized by hyponatremia with inappropriately concentrated urine (urine osmolality >100 mOsm/kg when serum osmolality is low) in a euvolemic patient with no other explanation for water retention. Key diagnostic criteria include: hypotonic hyponatremia, urine osmolality >100 mOsm/kg, euvolemia, urine sodium >40 mEq/L, and normal thyroid/adrenal function. Small cell lung cancer is the most common malignant cause of SIADH due to ectopic ADH production. The rate of sodium correction is critical - overly rapid correction can cause osmotic demyelination syndrome (central pontine myelinolysis), a devastating neurological condition.

Clinical Image

Diagnostic algorithm for hyponatremia showing the pathway to SIADH diagnosis based on volume status and urine studies.

Image Source: Wikimedia Commons - "Hyponatremia algorithm" License: CC BY-SA 4.0 URL: https://commons.wikimedia.org/wiki/File:Hyponatremia_Causes.svg

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