Cortisol, Urinary Free (24 hour) LabCorp

The Urinary Free Cortisol test reflects the portion of serum-free cortisol filtered by the kidney, and correlates well with cortisol secretion rate. The diagnosis of Cushing syndrome (CS) requries evidence of cortisol hypersecretion. While serum cortisol levels fluctuate unpredictably and are strongly dependent on concurrent cortisol-binding globulin (CBG) levels, a 25-hour urine specimen integrates the cortisol production for an entire day and is not affected by CBG. The most common causes of CS are pituitary adenoma (65% to 70%), adrenal tumor (15% to 20, and ectopic CS (10% to 15%). 

This test is not useful int he evaluation of adrenal insufficiency (AI). Results in AI overlap with normal ranges. Increased in pregnancy and with oral contraceptives. Increased excretion may be found with pseudo-Cushing syndrome, trauma, or infection. Tetracyclines may cause false elevation of results. 

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