My dogs have cushing's (aka hyperadrenocorticism) which involves the overproduction of the adrenal steroid, cortisol. Addison's (aka hypoadrenocorticism) is the exact opposite, meaning the adrenal glands are not producing any or enough cortisol. If not treated this can be a life threatening condition.
The symptoms you describe are commonly associated with Addison's but there are other conditions that share those symptoms as well. Therefore, most experienced vets will run a battery tests to rule out any other possibilities before doing an acth stimulation test, which is the gold standard in diagnosing Addison's. The test is done by drawing a sample for a baseline reading, a synthetic ACTH hormone (Cortrosyn) is given by injection, then a post blood sample is taken one hour later. Dogs with Addison's have a low cortisol concentration that will go unchanged even after the synthetic stimulation.
Once diagnosed, the condition can be successfully treated and a dog can live out their life expectancy with a good quality of life. Treatment consists of replacing the adrenal steroids with Prednisone (glucocorticoid) and Percorten-V or Florinef (mineralocorticoid).
Testing for Addison's disease - DVM
If your dog is ultimately diagnosed with Addison's, I highly recommend that you join a forum so that you can communicate with people that can share their knowledge and experience. I didn't do any research but I know there is a good support group at
AddisonDogs_ : AddisonDogs_
I hope this help.