Undetectable urinary free cortisol concentrations in a case of Cushing's disease.

Measurement of the 24-h urinary free cortisol is a valuable screening test of endogenous hypercortisolism and, although false positive results may occur in a few situations, for example endogenous depression, false negative results are unusual. We report a case of a 48-year-old lady with pituitary-dependent Cushing's disease, whose 24-h urinary free cortisol excretion was consistently undetectable in association with increased plasma and salivary cortisol concentrations and reduced dexamethasone suppressibility. The patient had chronic renal impairment (creatinine clearance 21 ml/min) as a consequence of hypertension, despite only modestly increased urea and creatinine concentrations. Urinary free cortisol measurements must be interpreted with caution in patients with renal impairment.