Cortisol (serum, 9am)

The range applies to a sample taken between about 6am and 10am (many labs print 133 to 537 or 166 to 507 nmol/L); late afternoon values are roughly half that, and there is no single range for a random sample.

Cortisol is the body's main steroid hormone, made by the adrenal glands under instruction from the pituitary. It keeps blood pressure and blood sugar steady, shapes the response to stress and illness, and follows a strong daily rhythm, highest shortly after waking and lowest around midnight. The blood test measures the total amount, most of which is bound to a carrier protein.

Doctors request a 9am sample when the adrenal glands may be underactive, for example with unexplained tiredness, weight loss, dizziness or low sodium, when someone has been on long-term steroid medicines, and as part of pituitary testing. Excess cortisol is usually looked for with other tests (late-night saliva, urine collection or a suppression test) rather than a single morning blood level.

• Stress, pain, illness, a difficult blood draw and lack of sleep
• The combined contraceptive pill, HRT and pregnancy, which raise the carrier protein and so raise total cortisol, often above the printed range
• Steroid medicines that cross-react with the assay, such as prednisolone and hydrocortisone
• Cushing's syndrome, where the body makes too much cortisol
• Depression, heavy alcohol use and poorly controlled diabetes
• Early morning sampling, as levels peak on waking

• Steroid medicines of any kind, including inhalers, creams, injections and tablets, which switch off the body's own cortisol
• Adrenal gland failure (Addison's disease)
• Pituitary or hypothalamus problems
• Afternoon or evening sampling
• Low carrier protein, for example in liver disease, kidney protein loss or severe illness
• Recent stopping of long-term steroid medicines

• The time of day is everything: the same person can read 500 at 8am and 150 at 5pm
• The combined pill or HRT commonly lifts total cortisol above the range without any adrenal problem
• Stress and a difficult blood draw raise it
• Any steroid medicine, including hydrocortisone cream or a steroid inhaler, lowers it or interferes with the assay

Taken between 8am and 9am, rested, and ideally not during illness. The time of the sample must be known to read it. Steroid medicines and oestrogen-containing contraception or HRT change the result, so the lab or doctor needs to know about them. If a morning value is unclear, a short stimulation test (Synacthen test) is done in clinic.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.