What Does Cortisol Do?
- Regulates the stress response ('fight or flight')
- Controls blood glucose levels
- Modulates immune and inflammatory responses
- Maintains blood pressure
- Regulates sleep-wake cycle (diurnal rhythm)
Interpreting Results
| Result | Likely Cause | Next Step |
|---|---|---|
| High morning cortisol | Stress, Cushing's syndrome, exogenous steroids | 24-hour urine free cortisol, dexamethasone suppression test |
| Low morning cortisol | Addison's disease, pituitary failure (secondary adrenal insufficiency) | Short Synacthen test (SST) |
| Loss of diurnal variation | Cushing's syndrome (midnight cortisol stays high) | Late-night salivary cortisol x2 |
| Normal AM cortisol despite symptoms | False negative — SST may still reveal impaired reserve | Clinical assessment + SST |
Timing Is CriticalCortisol must be taken at 8–9am for meaningful interpretation. A random sample has limited value unless it's very low (<100 nmol/L suggests adrenal insufficiency) or very high.
Cushing's Syndrome
Caused by excess cortisol — either from a pituitary tumour (Cushing's disease), adrenal tumour, or exogenous steroids. Features: central obesity, purple striae, moon face, easy bruising, hypertension, diabetes.
What causes high cortisol?
Chronic stress, obesity, depression, alcohol excess, Cushing's syndrome (pituitary or adrenal tumour), or long-term steroid medication use.
What is Addison's disease?
Autoimmune destruction of the adrenal cortex causing insufficient cortisol and aldosterone. Features: fatigue, weight loss, postural hypotension, bronze skin pigmentation, salt craving, low sodium.
What is the Short Synacthen Test?
A dynamic test that gives synthetic ACTH (synacthen) and measures cortisol at 0 and 30 minutes. A rise to >430 nmol/L rules out primary adrenal insufficiency.
Can cortisol levels be affected by stress?
Yes — acute physical or psychological stress significantly elevates cortisol. Results should be interpreted in the context of the patient's clinical state.
Medical Disclaimer: This page is for general education only and does not replace professional medical advice. Always consult a qualified healthcare provider.