Ninety-five percent of cortisol in the blood is bound to protein, principally to the cortisol binding globulin called transcortin. Consequently the amount of free cortisol that can be excreted in the urine unchanged is very small. Transcortin is almost fully saturated at normal cortisol concentrations and it follows that if cortisol production is increased, the concentration of free hormone, and thus the amount filtered at the glomerulus and excreted in the urine, increases at a rate similar to its rate of synthesis. For this reason, measurement of the twenty-four hour urinary excretion of cortisol, provided that an accurate urine collection can be made, is a sensitive way of detecting an increase in the secretion of the hormone, but not of decreased secretion. The measurement of urinary cortisol excretion is therefore valuable in the investigation of Cushing syndrome. As an indicator of adrenocortical function, measurement of blood cortisol concentrations is useful in the differential diagnosis of Addison and Cushing disease, hypopituitarism, and adrenal hyperplasia and carcinoma.
Anomalous cortisol concentrations have been demonstrated in patients with acute infections, severe pain, diabetes mellitus, heart failure, and in women either pregnant or on oestrogen therapy. Elevated steroid concentrations found in certain virilizing syndromes may interfere with cortisol measurement therefore the use of a highly specific antibody is vital in the measurement of cortisol.
The reference range for this test is 200nmol/24h.
London SE5 9RS
London SE5 9RS
Last updated: 04/09/2017