A 7-year-old male Shetland sheepdog presented to the Cornell University Hospital for Animals (CUHA) Emergency Service in January 2013 for evaluation of chronic anorexia and lethargy. In November 2012, the dog displayed clinical signs of increased appetite, thirst, and urination, and was diagnosed with hyperadrenocorticism (Cushing’s syndrome) by his primary veterinarian using an ACTH stimulation test. An abdominal ultrasound performed at CUHA revealed bilaterally enlarged adrenal glands. No treatment was initiated, primarily because the dog became anorexic approximately one week following the ACTH stimulation test. His polyuria and polydipsia also resolved at that time. On presentation to CUHA in January 2013, the dog was found to be hyperkalemic, hyponatremic, hypotensive, and dehydrated. These problems were confirmed to be due to primary hypoadrenocorticism (Addison’s disease). This case report will discuss the clinical presentation, diagnostic workup, treatment, and prognosis of hypoadrenocorticism, as well as theories regarding development of spontaneous hypoadrenocorticism following a diagnosis of hyperadrenocorticism.