After having diagnosed hyperadrenocorticism, (hypercortisolism, Cushing’s syndrome), an assay of the plasma level of adrenocorticotropic hormone (ACTH) is very useful for differentiating between the ACTH-dependent and ACTH-independent forms of the syndrome. The blood sample must be treated with particular care because ACTH is extremely sensitive to high temperatures and degrades quickly. Furthermore, it seems to adhere to glass test-tubes, unless these are silicon-coated. Generally, previously cooled, silicon-coated test-tubes containing EDTA are used to collect the blood sample and then kept on ice until processing. Samples sent to an external laboratory for processing must arrive at the laboratory frozen.
In the case of pituitary-dependent hypercortisolism (Cushing’s disease) and ectopic secretion of ACTH, the levels of the endogenous corticotropin are higher than physiological levels. These high levels occur in 85-90% of pituitary tumours, while 10-15% of subjects have levels that are not significant from a diagnostic point of view. In contrast, in the case of adrenal tumours or iatrogenic hypercortisolism, the concentration of ACTH in the blood is less than 10 pg/ml and frequently below the lower limit of detection of the test.