A modification of Murphy's competitive protein-binding radioassay for the measurement of urinary free corticoids in the low range is described. The modification includes the washing of the dichloromethane extract of urine, measurement of different-sized aliquots of the extract, and the use of a 3H-corticosterone labeled human plasma pool and Florisil adsorbent. These changes in the method are easily done and improve the precision of the assay enough to establish a lower limit of urinary free corticoid excretion in normal subjects. Urinary free corticoid excretion in normal subjects ranged between 16 and 71 μg/24 hr, with an average of 33 μg/24 hr. Patients with symptomatic adrenal insufficiency due to hypopituitarism, Addison's disease, or after withdrawal from long-term steroid therapy had values which ranged between 0 and 14 μg/24 hr. The validity of the results obtained by this direct extraction technique compares favorably with chromatographic methods using isotope dilution and double isotope derivative dilution techniques. The utility of the measurement of urinary free corticoid excretion is greatly increased by establishing a lower limit of excretion in normal subjects. This parameter of adrenal function should now be included as one of the important tests for the assessment of the physiologic status of the adrenal glands in patients suspected of having either hypercortisolism or adrenal insufficiency.
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Hsu et al. (1970) studied this question.