Radioimmunological determination of apparent free cortisol concentration: Some physiological and clinical applications

The measurement of free cortisol would be preferable with respect to the total hormone content, since it yields more reliable information about the plasma levels of the biologically active steroid.The methods so far described for the measurement of free cortisol in plasma use radiolabelled cortisol for determination of the steroid free fraction, and are generally unsuitable for routine use.We have developed a new method for the determination of the apparent free plasma cortisol concentration by means of direct radioimmunological measurement of dialyzed cortisol. This method is characterized by a sufficient degree of reproducibility and high sensitivity.Apparent free cortisol concentration in 40 control subjects of both sexes (blood drawn at 8 a.m.) was 9.00 ± 4.6 ng/ml. The mean value of free cortisol concentration in blood samples drawn at 11--12 p.m. from 21 of these subjects was highly significantly different (2.3 ± 1.6 ng/ml, p < 0.001). In addition, in 13 of these subjects circadian variation of the apparent free cortisol concentration showed a pattern similar to that of total cortisol concentration. The mean free cortisol concentration found in a group of women during normal pregnancy was significant higher than in non-pregnant women.Patients with renal insufficiency do not show a significant difference in free cortisol plasma levels, whereas higher values were found in hepatic cyrrhosis.