In a patient with hyponatremia (serum sodium 123 mmol/L), low serum osmolality and normal urine osmolality, could this be due to secondary adrenal insufficiency (Addison’s disease)?

Yes, this presentation is consistent with primary adrenal insufficiency (Addison's disease), but you must rule it out urgently with paired cortisol and ACTH measurements.

The laboratory pattern of hyponatremia (Na 123), low serum osmolality, and normal urine osmolality in this patient is highly suspicious for adrenal insufficiency and requires immediate diagnostic evaluation before attributing it to other causes like SIADH.

Why This Could Be Addison's Disease

The biochemical presentation you describe fits primary adrenal insufficiency (PAI):

• Hyponatremia is present in 90% of newly diagnosed PAI cases 1
• The mechanism involves sodium loss in urine combined with increased vasopressin and angiotensin II, which impair free water clearance 1
• Glucocorticoid deficiency causes inappropriately elevated vasopressin despite hypo-osmolality, creating a picture that mimics SIADH 2, 3

Critical Diagnostic Pitfall

The absence of hyperkalemia does NOT rule out Addison's disease. While the "classic" combination of hyponatremia and hyperkalemia is taught, potassium is only elevated in approximately 50% of PAI patients at diagnosis 1. In fact, if the patient has severe vomiting, they may even present with hypokalemia and alkalosis 1. A case report specifically documented a 10-year-old with rapidly progressive Addison's presenting with severe hyponatremia (112 mmol/L) and completely normal potassium 4.

Immediate Diagnostic Approach

Before diagnosing SIADH or any other cause of euvolemic hyponatremia, you MUST measure:

1. Paired serum cortisol and plasma ACTH (this is the diagnostic test for PAI) 1
2. In acute illness with suspected adrenal crisis:
   • Cortisol <250 nmol/L with elevated ACTH = diagnostic of primary PAI 1
   • Cortisol <400 nmol/L with elevated ACTH = strong suspicion of PAI 1

Why This Matters for Mortality

Treatment of suspected acute adrenal insufficiency should never be delayed by diagnostic procedures 1. If clinical suspicion is high (unexplained collapse, hypotension, vomiting, hyponatremia), start hydrocortisone 100 mg IV immediately, then draw cortisol and ACTH 5. The consequences of missing this diagnosis are severe:

• Adrenal crisis remains a cause of premature death 1
• Fluid restriction (the treatment for SIADH) is contraindicated and dangerous in adrenal insufficiency 6
• A retrospective study showed cerebral infarction developed in 21 of 26 SAH patients with hyponatremia who were fluid-restricted 6

Distinguishing From SIADH

The biochemical picture can be indistinguishable from SIADH 2, 3, 7:

• Both present with euvolemic hypotonic hyponatremia
• Both show inappropriately concentrated urine (though your patient has "normal" rather than high urine osmolality, which is actually more consistent with impaired free water clearance in adrenal insufficiency)
• Both have elevated urinary sodium

The original diagnostic criteria for SIADH specifically emphasized that normal adrenal reserve is essential for its diagnosis 2. However, studies show clinicians frequently ignore plasma cortisol measurement in hyponatremic patients, leading to underdiagnosis of glucocorticoid deficiency 2, 7.

Additional Clinical Clues to Look For

Physical examination findings that increase suspicion for PAI 1:

• Hyperpigmentation (gingival, skin creases, scars) - present in primary but not secondary AI
• Hypotension
• Missing or scanty pubic/axillary hair 7
• Pale, doughy skin 7
• Weight loss, fatigue, nausea/vomiting

Laboratory findings that support PAI 1:

• Mild eosinophilia
• Lymphocytosis
• Hypoglycemia (more common in children)
• Mild hypercalcemia (10-20% of cases)
• Increased plasma renin activity
• Low aldosterone and DHEAS

Secondary vs. Primary Adrenal Insufficiency

Your question mentions "secondary" adrenal insufficiency, but Addison's disease specifically refers to PRIMARY adrenal insufficiency (destruction of the adrenal glands themselves). Secondary adrenal insufficiency (pituitary/hypothalamic) also causes hyponatremia through the same mechanism of glucocorticoid deficiency 8, 2, 7, but:

• Secondary AI would show low or inappropriately normal ACTH (not elevated)
• Secondary AI typically does NOT cause hyperkalemia (mineralocorticoid function preserved)
• Secondary AI is often overlooked - one study found 28 cases of severe hyponatremia due to hypopituitarism, with 12 patients having recurrent admissions before diagnosis 7

Management Algorithm

1. If acute/symptomatic: Start hydrocortisone 100 mg IV immediately, then draw labs 5
2. Draw paired cortisol and ACTH before or immediately after first hydrocortisone dose
3. Do NOT fluid restrict - this patient needs isotonic saline if in crisis 5
4. If cortisol low and ACTH elevated → Primary AI (Addison's)
5. If cortisol low and ACTH low/normal → Secondary AI (pituitary/hypothalamic)
6. Once stabilized, perform ACTH stimulation test if diagnosis unclear 1

The key message: In any patient with euvolemic hyponatremia and concentrated urine, measure cortisol and ACTH before diagnosing SIADH. Missing adrenal insufficiency can be fatal, while the diagnostic test is simple and treatment is life-saving 1, 5, 2.