Urinary excretion of free cortisol in impaired renal function

Total cortisol and free, non protein-bound coritsol in plasma and urinary excretion of unconjugated free cortisol were measured during iv infusion of cortisol at varying dose rates in eight patients with impaired renal function. The results showed that free urinary cortisol decreased with decreased glomerular filtration rate (GFR), also compared to free cortisol level in plasma. An increase in free cortisol in plasma had no influence on GFR. It is concluded that determination of free urinary cortisol, otherwise useful in diagnosing Cushing's syndrome, may be of less value in patients with impaired renal function.