Primary adrenal insufficiency

A 32-year-old woman presents with generalised weakness, hyperpigmentation, weight loss, and postural hypotension. Blood tests show:

• Na⁺: 128 mmol/L (low)
• K⁺: 5.8 mmol/L (high)
• Cortisol: Low
• ACTH: High

What is the most likely diagnosis?

A) Addison’s disease
B) Cushing’s syndrome
C) Primary hyperaldosteronism
D) Pheochromocytoma
E) SIADH

Correct Answer:

A) Addison’s disease

Explanation:

• Primary adrenal insufficiency (Addison’s disease):
  • ↓ Cortisol → Fatigue, weight loss
  • ↓ Aldosterone → Hyponatraemia, hyperkalaemia, hypotension
  • ↑ ACTH → Hyperpigmentation
• B) Cushing’s syndrome → Incorrect. Would show hypertension, obesity, moon face, hyperglycaemia.
• C) Primary hyperaldosteronism → Incorrect. Would cause hypertension + hypokalaemia.
• D) Pheochromocytoma → Incorrect. Causes episodic hypertension, palpitations, sweating.
• E) SIADH → Incorrect. Causes hyponatraemia but no hyperkalaemia.

Key Tips for PLAB 1:

✅ Hyponatraemia + hyperkalaemia + hyperpigmentation = Addison’s disease.
✅ Confirm with Short Synacthen Test (low cortisol after ACTH stimulation).
✅ Treatment: Hydrocortisone + fludrocortisone.