What is cortisol?
Cortisol is a steroid hormone produced by the adrenal cortex (the outer layer of the adrenal glands, which sit on top of the kidneys). It follows a strong diurnal (daily) rhythm — highest in the early morning (peaking around 8am) and lowest at midnight. Cortisol regulates: blood sugar, blood pressure, anti-inflammatory response, metabolism of fats/proteins/carbohydrates, and the body's response to stress. Cortisol testing must be interpreted carefully in the context of the time of day and clinical situation.
Cortisol normal range
| Time | Normal Range (mcg/dL) | Notes |
|---|---|---|
| Morning (8am) | 10 – 20 | Peak; best time for testing |
| Afternoon (4pm) | 3 – 10 | Falls through the day |
| Evening / midnight | < 3 | Near baseline |
| Random | Varies widely | Time of sample must be noted |
HIGH Cortisol — Cushing Syndrome
Persistently elevated cortisol is called Cushing syndrome. Causes: long-term steroid medication (most common), a pituitary tumour producing excess ACTH (Cushing disease), an adrenal tumour producing excess cortisol, or an ectopic ACTH-producing tumour (lung, pancreas). Symptoms: central obesity (round face and belly with thin limbs), purple stretch marks (striae), easy bruising, high blood pressure, high blood sugar, muscle weakness, depression, and in women — irregular periods and hirsutism. Diagnosis involves 24-hour urine cortisol, late-night salivary cortisol and a 1mg dexamethasone suppression test.
LOW Cortisol — Adrenal Insufficiency
Low cortisol is called adrenal insufficiency. Primary adrenal insufficiency (Addison disease) means the adrenal glands are damaged — causes include autoimmune destruction (most common in developed countries), tuberculosis, HIV/AIDS, cancer and bilateral adrenal haemorrhage. Secondary adrenal insufficiency means the pituitary is not producing enough ACTH — most commonly caused by sudden withdrawal of long-term steroid therapy. Symptoms: profound fatigue, weight loss, low blood pressure, salt craving, nausea, darkening of the skin (primary only). An adrenal crisis (acute severe cortisol deficiency) is a medical emergency.
Questions to ask your doctor
- Is my cortisol taken at the right time of day?
- Do I need a short Synacthen test (cortisol stimulation test)?
- Should I have an ACTH level and a dexamethasone suppression test?
- If I am on long-term steroids, how do I safely reduce them?
- Do I need a pituitary or adrenal MRI/CT scan?