Evening Cortisol: Levels, Ranges, and Clinical Insights

What is Cortisol (Evening)?

Most people know cortisol as the 'stress hormone', but few realise that its natural rhythm — peaking in the morning and dropping at night — is a vital sign of adrenal health. An evening cortisol measurement helps distinguish Cushing's syndrome from normal variations. In my clinical practice, I often see patients who are surprised that a single blood draw at dusk can reveal so much about their adrenal axis.

Why Measure Evening Cortisol?

Cortisol follows a distinct diurnal pattern. In healthy individuals, levels are highest around 6–8 am and lowest around midnight. An evening sample (typically drawn between 4 pm and 6 pm) captures the descending limb. If evening cortisol remains high, it suggests loss of the normal negative feedback — a hallmark of Cushing’s syndrome or chronic stress. Conversely, extremely low evening values may point to adrenal insufficiency (Addison’s disease).

Normal Reference Ranges

Reference intervals vary by laboratory, assay method, and even posture. Below are typical ranges for a late-afternoon (16:00–18:00) serum cortisol. Always interpret results against your lab’s normal values.

Is High Evening Cortisol Dangerous?

Yes, persistently elevated evening cortisol — above 280 nmol/L (10 μg/dL) — should raise concern for Cushing’s syndrome, which carries risks of hypertension, diabetes, osteoporosis, and immune suppression. However, temporary elevations can occur with acute illness, depression, or even intense exercise. A single high reading warrants further testing, such as a 1 mg overnight dexamethasone suppression test or a 24-hour urinary free cortisol.

What Does Low Evening Cortisol Mean?

Evening cortisol <50 nmol/L (1.8 μg/dL) in adults may indicate adrenal insufficiency, especially if paired with symptoms like fatigue, weight loss, hyperpigmentation, or hypotension. In primary adrenal failure (Addison’s), cortisol is low throughout the day. In secondary insufficiency (pituitary or hypothalamic), the morning cortisol might be low as well. I always advise patients that a low evening value alone is not diagnostic — an ACTH stimulation test is the gold standard.

Factors Affecting Evening Cortisol

• Circadian disruption: Shift work, jet lag, or poor sleep can shift the nadir.
• Medications: Oral contraceptives, glucocorticoids (e.g., prednisone), and some antiepileptics alter binding proteins.
• Stress: Acute emotional or physical stress elevates cortisol — a single stressful event before drawing blood can raise the evening level.
• Pregnancy: Cortisol rises progressively due to placental CRH and oestrogen; evening values can double by the third trimester.

How to Prepare for the Test

No special fasting is required, but timing is crucial. The sample must be drawn between 16:00 and 18:00 (or as specified by your doctor). Avoid strenuous exercise, caffeine, and alcohol for 24 hours prior. If you are on any steroid medication (including inhalers or creams), inform your physician — they may need to adjust timing or withhold therapy.

When is This Test Ordered?

Your doctor may request evening cortisol if you have signs of Cushing’s syndrome (central obesity, facial rounding, purple striae) or adrenal insufficiency (chronic fatigue, dizziness, salt craving). It is also used to monitor treatment for adrenal disorders and to assess the circadian rhythm in patients with sleep disorders or chronic fatigue syndrome.