Cortisol Blood Test: What Your Results Mean

Cortisol's Daily Pattern: Why Timing Matters

Cortisol follows a strong circadian rhythm. It surges sharply in the 30–60 minutes after waking (the cortisol awakening response), peaks around 8 AM, and declines steadily throughout the day to its lowest point around midnight. The difference between AM and PM values is roughly 3–5 fold. A value that looks low in the morning is actually very high for midnight — context is everything.

Reference Ranges by Time of Day

Collection Time	Normal Range (US labs)
AM cortisol (6–9 AM, fasting)	10–20 µg/dL (275–550 nmol/L)
PM cortisol (3–5 PM)	3–10 µg/dL (82–275 nmol/L)
Late night / midnight	Below 1.8 µg/dL

Standard cortisol screening uses a morning blood draw before 9 AM, ideally fasting. Stress, pain, anxiety, or even the needle draw itself can transiently spike cortisol — a single elevated result should always be confirmed.

What Low Morning Cortisol Means

Morning cortisol below 3 µg/dL is a strong indicator of adrenal insufficiency and warrants urgent further testing. Values between 3–10 µg/dL are borderline and typically require an ACTH stimulation test (Synacthen test) to assess how well the adrenal glands respond to a chemical signal. Primary adrenal insufficiency (Addison's disease) presents with low cortisol alongside high ACTH — the pituitary is signalling hard but the adrenals cannot respond. Secondary adrenal insufficiency (pituitary failure) shows low cortisol with low or inappropriately normal ACTH.

Symptoms of adrenal insufficiency include profound fatigue, loss of appetite, weight loss, salt craving, nausea, low blood pressure, and — in primary Addison's — hyperpigmentation of skin folds and pressure areas. It is a serious condition requiring hormone replacement.

What High Cortisol Suggests

Cushing's syndrome (excess cortisol from any source — pituitary tumour, adrenal tumour, or long-term steroid medication) is the clinical concern with persistently elevated cortisol. Key features are loss of the normal diurnal rhythm, elevated midnight cortisol, failure to suppress on a 1 mg overnight dexamethasone suppression test, and elevated 24-hour urinary free cortisol. Cushing's is rare but causes central obesity, facial rounding, purple stretch marks, easy bruising, hypertension, diabetes, and bone loss. A single elevated AM cortisol — particularly if stress is present — is usually not Cushing's.

The Adrenal Fatigue Question

"Adrenal fatigue" — the concept that chronic psychological stress depletes the adrenal glands — is not a recognised medical diagnosis and lacks consistent evidence in published research. Studies measuring cortisol in people with burnout or chronic fatigue do not reliably show low cortisol. The symptoms attributed to adrenal fatigue (tiredness, brain fog, low motivation) more commonly reflect treatable conditions: thyroid disease, sleep apnoea, anaemia, depression, vitamin D or B12 deficiency. If your symptoms are real but cortisol tests are normal, investigate these other causes systematically.

Better Testing: Beyond a Single Blood Cortisol

A 4-point salivary cortisol test (collected at waking, noon, afternoon, and bedtime) provides a better picture of cortisol rhythm across the day than a single blood draw. A 24-hour urinary free cortisol measures total daily cortisol output and is the preferred initial screen for suspected Cushing's syndrome. Your doctor will recommend the appropriate test based on what they are investigating.