Cortisol (Urine)

General Information

Cortisol is the most significant glucocorticosteroid and is essential for the maintenance of several body functions. Like other glucocorticosteroids it is synthesised in the zona fasciculata of the cortex of the Adrenal gland. Cortisol is secreted in response to adrenocorticotrophic hormone (ACTH) which is released from the anterior pituitary gland. Cortisol is bound to cortisol-binding globulin and albumin with only the free (unbound) fraction being biologically active.

Cushing's syndrome is caused by prolonged exposure to elevated levels of either endogenous or exogenous glucocorticoids. Measurement of urinary free cortisol (UFC) in a 24 hour urine collection is frequently used as a first line screening test for Cushing's. It has the advantage in that it is simple for the patient to provide a sample and any increase in cortisol excretion should represent persistant elevation of circulating cortisol. This thereby avoids the problems inherent with interpreting serum cortisol values due to the ACTH-dependent circadian rhythm of cortisol secretion.

Due to problems with incomplete urinary collections and the false positives in UFC which occur in obesity and stressful illness, many clinicians may prefer an overnight or low dose dexamethasone suppression test to UFC measurement.