Urinary free cortisol versus 17-hydroxycorticosteroids: a comparative study of their diagnostic value in Cushing's syndrome.

We evaluated the usefulness of the basal urinary 24-h excretion rates of free cortisol versus 17-hydroxycorticosteroids in the diagnosis of Cushing's syndrome. On an outpatient basis, both urinary free cortisol and 17-hydroxycorticosteroids levels were determined in 48 patients with Cushing's syndrome, as well as in 95 obese and 94 healthy control persons of normal weight. Determination of the urinary free cortisol content allowed a clear-cut distinction between the patients with hypercortisolism and the controls, resulting in a sensitivity of 100% and specificity of 98% for the diagnosis of Cushing's syndrome. The diagnostic accuracy of urinary free cortisol was distinctly superior to that of 17-hydroxycorticosteroids, which showed a wide overlap of values between the groups, with a sensitivity of 73% and a specificity of 94%. In conclusion, the measurement of basal urinary free cortisol provided an excellent diagnostic sensitivity and specificity in the assessment of adrenocortical function. This simple and accurate test thus seems to be particularly useful in the outpatient evaluation of patients with suspected Cushing's syndrome.