What type of test is considered the most definitive for confirming Addison's disease?

The corticotropin stimulation test, also known as the ACTH stimulation test, is the most definitive test for confirming Addison's disease. This condition is characterized by an underproduction of cortisol due to insufficient stimulation of the adrenal glands. In this test, synthetic adrenocorticotropic hormone (ACTH) is administered, and the adrenal response is evaluated by measuring cortisol levels before and after administration.

In healthy individuals, ACTH stimulates the adrenal glands to produce cortisol, leading to a significant increase in cortisol levels. However, in patients with Addison's disease, the adrenal glands are damaged and unable to produce sufficient cortisol, resulting in little to no rise in cortisol levels following ACTH administration. This lack of response to ACTH is a hallmark of adrenal insufficiency and confirms the diagnosis of Addison's disease.

The other tests mentioned, such as thyroid function tests and cortisol level measurements, may provide valuable information but do not definitively diagnose Addison's disease. The 24-hour urine free cortisol test is primarily used for diagnosing Cushing's syndrome and assesses cortisol production, but it does not specifically evaluate adrenal response to ACTH, making it less relevant for Addison's disease diagnosis.