Special Precautions/Comments:

If also performing an overnight dexamethasone suppression test, please ensure the patient does not take dexamethasone during or just prior to the 24-hour collection as this may falsely lower the result.

Additional Information:

Urine free cortisol is measured in the initial investigation of Cushing’s syndrome (hypercortisolism). Cushing’s syndrome is characterised by hypercortisolism and therefore you would expect more free cortisol to be excreted in the urine. A 24-hour urine sample is required because cortisol secretion is diurnal, therefore a 24-hour collection is more representative than a random urine sample. 

The presence of common exogenous steroids (prednisolone and dexamethasone) are also measured and reported because these can affect the hypothalamic-pituitary-adrenal (HPA) axis. For example, they may result in suppression of cortisol production leading to a falsely low result. Conversely, prolonged use of exogenous steroids may cause Cushing’s syndrome (iatrogenic). 

Urine free cortisol is not recommended in patients with severe renal failure. Other investigations for Cushing’s syndrome include an overnight dexamethasone suppression test and a midnight salivary cortisol. Serum cortisol is subject to diurnal variation and so random measurement is not useful in the investigation of Cushing’s syndrome.