Which test is used as a screening test for primary hypercortisolism?

• A. 24-hour Urine Free Cortisol
• B. CRH stimulation
• C. Midnight plasma cortisol
• D. Low-dose dexamethasone suppression

Answer: (A)

Screening for primary hypercortisolism aims to detect sustained excess cortisol production across the day, not just a single snapshot. The 24-hour urine free cortisol test does this by capturing all cortisol excreted in urine over a full day, so it reflects total daily production rather than the diurnal fluctuations or transient spikes seen with a single blood sample. In hypercortisolism, cortisol output is chronically elevated, so the amount of free cortisol in urine over 24 hours rises, making this test a practical and reliable first screen. It’s largely independent of acute stress or pulsatile secretion that can skew serum measurements, and it directly measures the biologically active, free cortisol. Of course, accurate collection is key and factors like incomplete urine collection or kidney issues can affect results, but when done properly it provides a strong initial indication that further confirmatory testing is warranted.