Serum Cortisol
Also known as: Serum cortisol, Morning cortisol, Random cortisol
Cortisol is the body's primary stress hormone — essential for blood sugar regulation, immune response, and blood pressure. Testing requires strict attention to timing because cortisol follows a powerful circadian rhythm.
Venous blood (serum separator tube)
4–8 hours
Not required
Hormones
Reference Ranges
| Measurement | Reference Range | Unit |
|---|---|---|
| Morning (8–9am) | 6 – 23 | mcg/dL |
| Evening (3–4pm) | 2 – 11 | mcg/dL |
Reference ranges are typical adult values. Your laboratory may use slightly different intervals — always use your lab's reported reference.
What This Test Measures
Cortisol is produced by the adrenal cortex under control of ACTH from the pituitary. It peaks within 30–60 minutes of waking (the 'cortisol awakening response'), drives gluconeogenesis, mobilises fats, modulates immunity, and maintains blood pressure. A single random cortisol is difficult to interpret without knowing the time of draw — the circadian pattern is essential context.
When Is This Test Ordered?
- Suspected adrenal insufficiency (Addison's disease) — fatigue, hypotension, hyperpigmentation
- Suspected Cushing's syndrome — central obesity, striae, easy bruising, hypertension
- Evaluating adrenal crisis in acutely unwell patients
- Monitoring adrenal function post-steroid therapy
When Results Are High
Persistently high cortisol (Cushing's syndrome) causes: central obesity, facial fullness, thin skin, purple striae, muscle weakness, hypertension, diabetes, and osteoporosis. The overnight dexamethasone suppression test (1 mg dex at midnight, cortisol drawn at 8am) is the standard screening test — normal cortisol suppresses below 1.8 mcg/dL.
When Results Are Low
Morning cortisol below 3 mcg/dL is highly suggestive of adrenal insufficiency; above 18 mcg/dL effectively rules it out. The grey zone (3–18 mcg/dL) requires an ACTH stimulation test to confirm or exclude adrenal insufficiency.
Factors That Can Affect Your Result
- Time of draw — cortisol at 9am is 3–5× higher than at 9pm
- Physical stress (illness, surgery, trauma) can raise cortisol 5–10×
- Exogenous steroids suppress cortisol — any topical, inhaled, or oral steroid counts
- Oestrogen (oral contraceptives, HRT) raises cortisol-binding globulin, elevating total cortisol without increasing free active cortisol
Tracking This Test Over Time
A single cortisol rarely diagnoses adrenal disease — the pattern and context matter far more. If Cushing's is suspected, the gold-standard screening involves a late-night salivary cortisol (which should be low) plus the overnight dexamethasone suppression test. These two tests together have over 95% sensitivity.
Track your Cortisol in LoomsRelated Tests
Sodium
Sodium is the primary electrolyte that determines blood osmolality and fluid distribution. Both low and high sodium levels can cause severe neurological symptoms and require careful correction.
Potassium
Potassium is the most important intracellular electrolyte and plays a critical role in heart rhythm and muscle function. Even small deviations outside the reference range can cause life-threatening cardiac arrhythmias.
Medical disclaimer
This page is for educational purposes only and does not constitute medical advice. Reference ranges vary between laboratories, and results must be interpreted in the context of your symptoms, history, and other tests. Always discuss your results with your healthcare provider. Content reviewed by the Looms medical team, 2026-06-01.