Possible causes of low cortisol are e.g. hypoadrenocorticism (Addison’s disease). There is a circadian rhythm of cortisol secretion in the normal population, so whole blood cortisol concentration is measured by blood collection at 8 a.m. (peak secretion) and 4 p.m. (reduced cortisol levels). Normal reference values for plasma cortisol range from 137.95 to 634.57 mmol/L at 8 a.m. and 82.77 to 413.85 mmol/L at 4 p.m., with 4 p.m. less than 50% of the 8 a.m. level; and urinary free cortisol ranges from 55 to 250 mmol/24 hours. Patients with low cortisol should go to the hospital and be treated under doctor’s supervision.