Free urinary cortisol radioimmunoassay

Summary The availability of a specific antiserum has made it possible to develop in our laboratory a radioimmunoassay for free urinary cortisol which is quite simple and rapid to perform. No preliminary procedures of chromatographic purification are necessary, and no correction for losses is required. Precision and accuracy tests are satisfactory. Seventeen normal subjects, 10 obese subjects, 18 patients with Cushing's syndrome, 4 hypopituitaric patients and 4 with Addison's disease were studied. The values of free urinary cortisol were compared with those of urinary 17-OHCS, plasma cortisol and, in some cases, the cortisol secretion rate. In normal subjects, the mean free urinary cortisol was 77.22±7.74 μg/24 h, in obese subjects it was 112.05±13.64 μg/24 h, and in patients with Cushing's syndrome it was at significantly higher levels, with a mean value of 488.06±64.39 μg/24 h. There was no overlap between the values obtained in the first two groups and those of subjects with adrenal hyperfunction, and the tesults were similar to those shown by the cortisol secretion rate. The same was not true for urinary 17-OHCS and plasma cortisol. Thus, the assay of free urinary cortisol, that is the free biologically active circulating fraction, may provide a relatively simple alternative to measurement of the cortisol secretion rate, as a single differential test for the diagnosis of conditions with adrenal hyperfunction. Less satisfactory were the results obtained in patients with primary and secondary adrenal hypofunction. Another limitation of the method is its relative non-specificity, since the results of 16 urinary samples measured without preliminary chromatography were 29.6% higher than those after previous chromatographic purification. However, in our experience, this overestimation does not affect the value of the method as a screening test for the diagnosis of Cushing's syndrome.