What is the common etiology of low Thyroid-Stimulating Hormone (TSH) with concurrent hyponatremia and hypotension after recent pleural mass resection?

Adrenal Insufficiency Following Pleural Mass Resection

The most common etiology of low TSH with concurrent hyponatremia and hypotension after recent pleural mass resection is adrenal insufficiency (secondary or tertiary), which can occur postoperatively due to surgical stress, hemorrhage, or unmasking of pre-existing adrenal dysfunction.

Clinical Context and Pathophysiology

The triad of low TSH, hyponatremia, and hypotension in the postoperative setting strongly suggests central adrenal insufficiency (secondary or tertiary) rather than primary adrenal failure. 1 This presentation occurs because:

• Surgical stress can unmask subclinical pituitary or hypothalamic dysfunction
• Perioperative hemorrhage or hypotension may cause pituitary infarction (Sheehan-like syndrome in non-obstetric settings)
• The low TSH indicates central hypothyroidism (pituitary/hypothalamic origin), which commonly coexists with central adrenal insufficiency as part of hypopituitarism 2

Diagnostic Approach

Essential Laboratory Evaluation

Immediate tests to confirm adrenal insufficiency: 3

• Morning cortisol level (8 AM): <3 μg/dL confirms adrenal insufficiency; 3-15 μg/dL requires ACTH stimulation test
• ACTH level: Low or inappropriately normal ACTH with low cortisol confirms central adrenal insufficiency
• Free T4: Will be low in central hypothyroidism despite low/normal TSH 2
• Serum osmolality: Typically <275 mOsm/kg with hyponatremia 3
• Urine osmolality and sodium: Inappropriately elevated (>100 mOsm/kg urine osmolality, >20 mEq/L urine sodium) 3

Critical Distinction from SIADH

While the pleural mass raises concern for paraneoplastic SIADH (particularly if small cell lung cancer), several features distinguish adrenal insufficiency: 3, 4

• Hypotension is prominent in adrenal insufficiency but uncommon in SIADH
• Low TSH suggests pituitary dysfunction, not isolated SIADH
• SIADH diagnosis requires exclusion of adrenal insufficiency and hypothyroidism 3
• In SIADH, patients are typically euvolemic without hypotension 4

Immediate Management

Acute Treatment Protocol

For suspected adrenal crisis with hypotension: 1

1. Do NOT wait for laboratory confirmation - treat empirically if clinical suspicion is high
2. Administer hydrocortisone 100 mg IV bolus immediately, followed by 50-100 mg IV every 6-8 hours
3. Aggressive volume resuscitation with isotonic saline (0.9% NaCl) at 1-2 L over first 1-2 hours 1
4. Correct hypotension first - sodium will often improve with cortisol replacement and volume expansion

Sodium Correction Guidelines

Critical safety parameters: 1

• Maximum correction rate: 8 mmol/L in 24 hours to prevent osmotic demyelination syndrome
• High-risk patients (malnutrition, alcoholism, liver disease): limit to 4-6 mmol/L per day 1
• Monitor sodium every 2-4 hours during active correction 1

Alternative Considerations

If Lung Cancer is Confirmed

SIADH remains possible but requires: 3, 4, 5

• Exclusion of adrenal insufficiency (normal cortisol response)
• Exclusion of hypothyroidism (normal free T4)
• Euvolemic state on examination
• Urine osmolality >300 mOsm/kg with serum osmolality <275 mOsm/kg 4

Treatment for confirmed SIADH: 3, 4

• Fluid restriction to <1 L/day for mild cases
• 3% hypertonic saline for severe symptomatic hyponatremia (<120 mEq/L with neurological symptoms)
• Target correction of 6 mEq/L over 6 hours, maximum 8 mmol/L in 24 hours 4

Cerebral Salt Wasting (Unlikely but Consider)

If there was CNS involvement from metastases or surgical complications: 1

• Presents with true hypovolemia (unlike SIADH)
• Requires volume and sodium replacement, NOT fluid restriction
• Treatment includes isotonic/hypertonic saline plus fludrocortisone 1

Common Pitfalls to Avoid

1. Never restrict fluids in a hypotensive patient - this suggests hypovolemia requiring volume expansion 1
2. Never ignore low TSH - always check free T4 to assess for central hypothyroidism 2
3. Never diagnose SIADH without excluding adrenal insufficiency and hypothyroidism first 3
4. Never correct sodium >8 mmol/L in 24 hours - risk of osmotic demyelination syndrome is catastrophic 1
5. Never delay empiric hydrocortisone if adrenal crisis is suspected - waiting for labs can be fatal

Monitoring and Follow-up

During acute management: 1

• Serum sodium every 2-4 hours until stable
• Blood pressure and hemodynamic status continuously
• Daily cortisol and ACTH once stable
• Free T4 and TSH to guide thyroid replacement

Long-term considerations:

• If central adrenal insufficiency confirmed, patient requires lifelong glucocorticoid replacement
• Stress-dose steroids needed for future surgeries or illnesses
• Evaluate for other pituitary hormone deficiencies
• MRI pituitary if central hypopituitarism confirmed