Cortisol

The normal reference range for Cortisol is Cortisol follows a strong daily rhythm, so timing is everything. A morning (around 8-9am) serum cortisol is typically 140-700 nmol/L, falling to under ~165 nmol/L by late evening. Because of this rhythm, the time of the draw must be recorded for the result to mean anything.. Reference ranges may vary between laboratories, so always consult your healthcare provider for interpretation.

Helps assess adrenal function and stress response, with implications for energy, immunity, and metabolic health.

High cortisol most often reflects physical or emotional stress, acute illness, pregnancy or oestrogen (the pill raises cortisol-binding protein), and recent strenuous exercise. Persistently high levels that lose their daily rhythm raise concern for Cushing's syndrome — caused by long-term corticosteroid medication (the most common cause), a pituitary tumour producing ACTH (Cushing's disease), or an adrenal tumour. Depression and poorly controlled diabetes can also mildly elevate cortisol.

Low cortisol can indicate adrenal insufficiency. Primary adrenal failure (Addison's disease) is often autoimmune and is accompanied by low sodium, high potassium, fatigue, weight loss and skin pigmentation. Secondary adrenal insufficiency results from pituitary disease or, very commonly, abrupt withdrawal from long-term steroid medication, which suppresses the body's own production. Low cortisol with these features needs prompt assessment.

A baseline cortisol is taken in the morning, between 8 and 9am, when levels peak. Abnormal results are not diagnostic on their own and are followed by dynamic tests — a Synacthen (ACTH stimulation) test for suspected insufficiency, or dexamethasone suppression and 24-hour urinary/late-night salivary cortisol for suspected excess.