Which endocrine disorders can cause hyponatremia, and what laboratory tests should be ordered to evaluate the cause?

Endocrine Causes of Hyponatremia and Laboratory Evaluation

Primary adrenal insufficiency (Addison's disease) and secondary adrenal insufficiency (hypopituitarism) are the most critical endocrine causes of hyponatremia that must not be missed, while severe hypothyroidism rarely causes hyponatremia and should only be considered after excluding adrenal pathology. 1, 2

Primary Endocrine Disorders Causing Hyponatremia

1. Primary Adrenal Insufficiency (Addison's Disease)

This is the most important endocrine cause to identify, as missing it can be fatal. Hyponatremia occurs in 90% of newly presenting cases 1. The mechanism involves:

• Sodium loss in urine due to aldosterone deficiency
• Elevated vasopressin and angiotensin II impairing free water clearance
• The classic combination of hyponatremia with hyperkalemia occurs in only about 50% of patients 1

Key clinical clues:

• Hyperpigmentation (specific sign)
• Salt craving
• Hypotension with postural drop
• Unexplained collapse, vomiting, or diarrhea
• Weight loss and fatigue 1, 3

2. Secondary Adrenal Insufficiency (Hypopituitarism)

This presents more subtly than primary adrenal insufficiency and requires high clinical suspicion 4, 5. Hyponatremia occurs without hyperkalemia (since aldosterone production is preserved) 2.

Key clinical clues:

• Headache (85% in hypophysitis)
• Visual changes (less common)
• History of pituitary tumor, surgery, or cranial radiation
• Absence of hyperpigmentation 4

3. Severe Hypothyroidism/Myxedema

Critical caveat: Recent evidence shows hypothyroidism rarely causes hyponatremia in uncomplicated cases 6, 7. Hyponatremia occurs primarily in severe hypothyroidism and myxedema, not mild-to-moderate disease. When hyponatremia is present with mild hypothyroidism, always search for alternative causes (drugs, infections, adrenal insufficiency) 6.

Essential Laboratory Workup

When evaluating hyponatremia, perform this algorithmic approach:

Step 1: Initial Assessment 1, 2, 3

• 8 AM cortisol and ACTH (mandatory - do not skip)
• TSH and free T4
• Serum and urine osmolality
• Urine sodium and electrolytes
• Basic metabolic panel (assess for hyperkalemia)

Step 2: Interpretation for Adrenal Insufficiency 1, 2

Primary Adrenal Insufficiency:

• Cortisol <250 nmol/L (<9 μg/dL) with elevated ACTH in acute illness = diagnostic
• Cortisol <400 nmol/L (<14.5 μg/dL) with elevated ACTH = strong suspicion
• If equivocal: ACTH stimulation test (cosyntropin 0.25 mg) - peak cortisol <500 nmol/L (<18 μg/dL) confirms diagnosis

Additional tests once primary adrenal insufficiency confirmed:

• Plasma renin activity (elevated)
• Aldosterone (low)
• DHEA-S (low)

Secondary Adrenal Insufficiency:

• Low cortisol with normal or low ACTH
• If ACTH stimulation test is normal but suspicion remains high: insulin hypoglycemia test or metyrapone test (not in acute situations) 2

Step 3: Hypopituitarism Evaluation 4

If secondary adrenal insufficiency suspected, assess other pituitary axes:

• FSH, LH
• Testosterone (men) or estradiol (women)
• IGF-1
• MRI sella with pituitary cuts

Step 4: Thyroid Assessment 6, 7

• TSH with free T4
• Only consider hypothyroidism as the cause if:
  • TSH markedly elevated (typically >10 mIU/L)
  • Free T4 significantly low
  • Clinical features of myxedema present
  • Adrenal insufficiency has been excluded first

Critical Pitfalls to Avoid

1. Never assume SIADH without excluding adrenal and thyroid disease 8, 9. Approximately two-thirds of patients with hyponatremia never have cortisol measured 9.

2. Do not delay treatment in suspected acute adrenal crisis 1, 2. Draw cortisol and ACTH, then immediately start hydrocortisone without waiting for results.

3. In hypopituitarism with both adrenal insufficiency and hypothyroidism, always start steroids before thyroid hormone to avoid precipitating adrenal crisis 4.

4. Hyperkalemia is absent in secondary adrenal insufficiency 2. Its presence points to primary adrenal disease.

5. Mild hypothyroidism (TSH 4-10 mIU/L) can occur in primary adrenal insufficiency due to lack of cortisol's inhibitory effect on TSH 1. This resolves with cortisol replacement.

6. Do not attribute hyponatremia to mild-moderate hypothyroidism - search for other causes 6, 7.

Practical Algorithm Summary

For any patient with hyponatremia:

1. Measure 8 AM cortisol + ACTH (non-negotiable)
2. Check TSH + free T4
3. If cortisol <400 nmol/L or ACTH elevated → pursue adrenal workup urgently
4. If cortisol adequate but TSH >10 with very low T4 and myxedema features → consider hypothyroidism
5. If both normal → evaluate for SIADH and other causes

The prevalence of adrenal insufficiency in hyponatremia is low (0.7%) 9, but the consequences of missing it are severe, making routine screening essential in the diagnostic algorithm 10.