A case of low cortisol-binding globulin: use of plasma free cortisol in interpretation of hypothalamic-pituitary-adrenal axis tests.

We investigated a patient with suspected hypopituitarism who showed subnormal cortisol responses to stimulation tests with adrenocorticotrophic hormone (ACTH) and hypoglycaemia, but normal ACTH and 11-deoxycortisol responses in the metyrapone test. Cortisol-binding globulin (CBG) was measured by enzyme-linked immunosorbent assay (ELISA), and was used to calculate plasma free cortisol and free cortisol index. The patient had a low plasma CBG concentration. Reinterpreted in terms of free cortisol and free cortisol index, his responses to ACTH were normal. We conclude that despite subnormal total cortisol responses to ACTH and hypoglycaemia, the patient had a normal hypothalamic-pituitary-adrenal (HPA) axis. Measurement of CBG concentration and calculation of free cortisol or free cortisol index can help avoid false interpretations of dynamic tests of the HPA axis based on plasma total cortisol.