Which laboratory test is used to diagnose adrenal insufficiency?

The ACTH stimulation test is primarily used to diagnose adrenal insufficiency because it assesses the adrenal glands' ability to respond to adrenocorticotropic hormone (ACTH). In this test, synthetic ACTH is administered, and the subsequent cortisol level is measured. In healthy individuals, this stimulation should lead to an increase in cortisol production from the adrenal glands. However, in cases of adrenal insufficiency—whether due to primary adrenal gland failure (Addison's disease) or secondary causes—the adrenal glands will not produce adequate cortisol in response to ACTH stimulation.

This test is particularly valuable because it helps to differentiate between primary adrenal insufficiency, where there is a damage to the adrenal glands, and secondary adrenal insufficiency, caused by insufficient ACTH production from the pituitary gland. The nuanced response measured in the ACTH stimulation test is integral to establishing the diagnosis and guiding treatment.

Other tests, such as measuring cortisol levels at specific times of day or conducting a blood glucose test, can provide useful information but do not specifically confirm the diagnosis of adrenal insufficiency on their own.