Cortisol Testing for Weight Loss Evaluation
For evaluating unexplained weight loss, order a morning (8 AM) serum cortisol level first; if low (<140 nmol/L or <5 μg/dL), measure plasma ACTH simultaneously and confirm with a corticotropin (cosyntropin) stimulation test to diagnose adrenal insufficiency, which can cause weight loss and carries significant mortality risk if untreated.
Clinical Context and Reasoning
When weight loss is the presenting concern, you're primarily worried about adrenal insufficiency (specifically primary adrenal insufficiency), which causes weight loss along with fatigue, hypotension, and electrolyte abnormalities. This is a life-threatening condition that requires prompt diagnosis 1.
Initial Testing Algorithm
Step 1: Morning Cortisol + ACTH
- Draw at 8 AM (though less critical than previously thought)
- If cortisol <140 nmol/L (5 μg/dL) **AND** ACTH elevated >2-fold above upper limit of normal, diagnosis is highly likely 1
- ACTH >66 pmol/L represents maximal adrenal stimulation 1
Step 2: Confirmatory Testing
- Proceed with standard-dose corticotropin stimulation test (250 μg cosyntropin IV or IM)
- Measure cortisol at baseline, 30 minutes, and 60 minutes
- This is the diagnostic "gold standard" with 92% sensitivity for primary adrenal insufficiency 1
- Can be performed at any time of day without concern for diurnal variation 1
Important Clinical Pitfalls
Do NOT order tests for Cushing's syndrome (like dexamethasone suppression test or 24-hour urinary free cortisol) when evaluating weight loss. Cushing's causes weight gain, not weight loss. The research evidence showing changes in cortisol-binding globulin with weight loss 2, 3 is relevant to obesity research but not to your clinical question about evaluating a patient presenting with weight loss.
Critical caveat: If the patient appears acutely ill with hypotension, hyponatremia, hyperkalemia, or hypoglycemia, start IV hydrocortisone immediately at stress doses without waiting for test results 1. Delayed treatment increases mortality. You can still interpret cortisol and ACTH levels drawn before giving hydrocortisone.
Why This Approach
The Endocrine Society guidelines 1 emphasize that adrenal insufficiency is a diagnostic emergency with high morbidity and mortality if missed. The combination of low morning cortisol with elevated ACTH provides strong diagnostic evidence, and the cosyntropin stimulation test offers definitive confirmation with high validation against the insulin tolerance test.
Additional consideration: Check DHEAS levels as well—levels well below normal for age and sex provide a useful early sign of primary adrenal insufficiency, though they cannot diagnose it in isolation 1.
The low-dose (1 μg) cosyntropin test offers no diagnostic advantage over the standard 250 μg dose 1, so use the standard dose which has better validation and wider availability.