Metabolic Fate of Cortisol	

At normal plasma concentrations, approximately ninety per cent of cortisol is bound to the specific binding protein - corticosteroid- binding globulin (CBG). The average production rate of cortisol is 20mg/d. Cortisol is metabolised rapidly, mainly by the liver, and has a plasma half-life of approximately two hours; the metabolic clearance rate is 200l/d. The major metabolites are formed by reduction of the double bond and ketone groups to produce tetrahydrocortisol, tetrahydrocortisone, cortol, and cortolone (Fig. 7.13). These are excreted into the urine in conjugation with glucuronic acid and account for approximately sixty to seventy per cent of the total cortisol produced. Estimation of these steroids in a twenty-four-hour urine sample thus provides an estimate of the secretion rate of cortisol. A small amount of cortisol (up to 150µg/d) is excreted unchanged and represents the unbound cortisol in plasma which is filtered through the kidney.