The corticotrophin-releasing hormone (CRH) test has become established as a powerful tool in the investigation of ACTH-dependent Cushing's syndrome: l00┬╡g of synthetic CRH is administered intravenously, with measurement of ACTH and cortisol levels being made every fifteen minutes for two hours. Patients with ectopic ACTH secretion fail to respond to stimulation with CRH, while in Cushing's disease, serum cortisol levels usually rise excessively (Fig. 8.14); some ten per cent of patients with Cushing's disease show an uncharacteristic lack of response.
The combination of a high-dose dexamethasone suppression test with a CRH test identifies over ninety-five per cent of patients with Cushing's disease.
