How is Addison's disease diagnosed?
In suspected cases of Addison's disease, one needs to demonstrate that adrenal hormone levels are low after appropriate stimulation with synthetic pituitary hormone. Once demonstrated, the cause of adrenal failure needs to be elucidated. The most common cause is autoimmune, and can be tested for with an assay for 21-hydroxylase antibodies. If there are no
antibodies present, infectious or genetic causes should be sought. This may include imaging of the adrenal glands, tests for tuberculosis or HIV infection, and searching for metastatic cancer.
Many patients do not recognize the slow progression of symptoms and the disease is ultimately identified when a physician notices the areas of increased pigmentation of the skin. Once suspected, a number of blood tests can lead to the diagnosis of Addison's disease. It is not sufficient to demonstrate low blood cortisol levels, as normal levels of cortisol vary quite widely. Instead, patients are given a testing dose of another hormone called corticotropin (ACTH). ACTH is produced in the body by the pituitary gland, and normally acts by promoting growth within the adrenal cortex and stimulating the production and release of cortisol. In Addison's disease, even a dose of synthetic ACTH does not increase cortisol levels.
To distinguish between primary adrenocortical insufficiency (Addison's disease) and secondary adrenocortical insufficiency (caused by failure of the pituitary to produce enough ACTH), levels of ACTH in the blood are examined. Normal or high levels of ACTH indicate that the pituitary is working properly, but the adrenal cortex is not responding normally to the presence of ACTH. This confirms the diagnosis of Addison's disease.