Diminished urinary free cortisol excretion in patients with moderate and severe renal impairment.

Abstract

The diagnosis of Cushing syndrome remains a challenge for most general clinicians and even endocrinologists. Because the clinical features of Cushing syndrome overlap with those in some healthy obese individuals, biochemical investigations play an important role. The 24-h urinary free cortisol excretion is widely used because of its relatively good sensitivity and specificity (1)(2). Although salivary cortisol has been recommended for screening of Cushing syndrome (3), this assay has not been widely available. Because plasma free cortisol is filtered through the glomeruli with partial tubular reabsorption, the amount of free cortisol appearing in the urine is theoretically dependent on the glomerular filtration rate. However, the high reliability of using 24-h urinary cortisol excretion for the diagnosis of Cushing syndrome implies that urinary excretion of cortisol is relatively unaffected by renal function. In patients with confirmed Cushing disease and severe renal impairment, the urinary free cortisol excretion rate reportedly is normal despite markedly increased plasma cortisol (4)(5)(6). The relationship between glomerular filtration rate and urinary cortisol excretion has not been documented in patients with different degrees of renal impairment. We selected 100 leftover portions of urine samples that had been sent for the measurement of creatinine clearance (CrCl) in the Prince of Wales Hospital of Hong Kong during September, October, and November 2002. All patients were given clear instructions by an experienced nurse to ensure the completeness of 24-h urine collection. Simultaneous serum samples were also received for the …