Test Background Cortisol is secreted in a diurnal pattern with peak values occurring in the early morning. Random serum cortisol may be difficult to interpret. The diurnal rhythm may be disrupted or lost in Cushing’s syndrome. Free cortisol measured in a 24 hour urine collection may be a more useful diagnostic tool in some patients.
Clinical Indications Most specific diagnostic indicator of Cushing’s syndrome Differential diagnosis of phaeochromocytoma and Conn’s syndrome