The free cortisol index.

The development of the Free Cortisol Index, an indirect measurement of the free or metabolically active portion of circulating cortisol, is described. Patients with non-adrenal disease had FCl values in the same range as normal subjects and in patients with adrenal disease the FCl values moved in the same direction as the total plasma cortisol. FCl values exhibited the same diurnal variation and response to ACTH as did total plasma cortisol in normal subjects. In pregnancy a significant rise in FCl was found confirming previous evidence of a degree of hyperadrenocorticalism in this condition.