Cortisol

What it measures

Cortisol is the main glucocorticoid made by the adrenal glands, with a daily peak in the early morning. It is measured to detect adrenal insufficiency and, with dynamic tests, Cushing syndrome.

Specimen and preparation

• Specimen: Serum; also late-night saliva or 24-hour urine free cortisol
• Tube: Gold top
• Preparation: Record the collection time; morning (8–9 am) sample for insufficiency. Oral oestrogens raise total cortisol; prednisolone cross-reacts.

Reference range

Reference ranges differ between laboratories, methods and populations. Always use the range printed on your own lab report.

Causes of high values

• Cushing syndrome
• Stress, illness and pregnancy
• Oral contraceptives (raised binding globulin)
• Depression and alcohol excess

Causes of low values

• Addison disease
• Pituitary failure (secondary insufficiency)
• Long-term steroid withdrawal
• Congenital adrenal hyperplasia

Method

Chemiluminescent immunoassay or LC-MS/MS

Good to know

A morning cortisol below about 3 µg/dL (≈83 nmol/L) strongly suggests adrenal insufficiency and above about 15 µg/dL (≈400 nmol/L) makes it unlikely; values in between need a Synacthen (ACTH stimulation) test. Cut-offs are assay-specific.

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