A radioimmunoassay for 18-hydroxycortisol in plasma and urine

Increased excretion of 18-hydroxycortisol has been proposed as a specific biochemical marker for differential diagnosis of primary aldosteronism. We describe the development of a direct RIA with an 125I label that permits measurement of the steroid in less than or equal to 0.5 microL of urine or less than or equal to 25 microL of plasma. For control subjects, the mean concentrations of 18-hydroxycortisol in urine and plasma were 310 (SD 178) nmol/24 h (n = 32) and 2.27 (SD 0.80) nmol/L (n = 37), respectively; patients with Conn's adenoma or glucocorticoid-remediable aldosteronism had values for urine in the range 1084 to 6534 nmol/24 h and concentrations in plasma ranging from 6.49 to 31.20 nmol/L. Patients with idiopathic zona glomerulosa hyperplasia had values for urine and plasma ranging from 353 to 734 nmol/24 h and from 0.26 to 6.60 nmol/L, respectively. Concentrations of 18-hydroxycortisol in urine clearly discriminate patients with idiopathic hyperplasia from those with other forms of primary aldosteronism, but further work is required to assess the diagnostic accuracy of determinations in plasma.