The first test for diagnosing Addison's disease is typically a blood test to measure the levels of certain hormones, such as cortisol and aldosterone, which are produced by the adrenal glands. If the levels of these hormones are found to be lower than normal, it may indicate that the adrenal glands are not functioning properly and that the patient may have Addison's disease. Other tests may be required to confirm the diagnosis, such as a CT scan or MRI to view the adrenal glands, or an ultrasound to measure the size of the adrenal glands. Additionally, a 24-hour urine collection test may be ordered to measure the levels of cortisol and other hormones in the body. Ultimately, the diagnosis of Addison's disease is made when the blood tests, imaging studies, and other tests all point to an underlying adrenal gland disorder.