Hyperadrenocorticism or Cushing’s syndrome occurs when a dog is chronically exposed to high levels of the hormone cortisol. Cortisol is produced by 2 small glands situated near the kidneys known as the adrenal glands. The production and release of cortisol from the adrenal glands is controlled by the hormone ACTH (adrenocorticotrophic releasing hormone). ACTH is produced by a pea-sized gland at the base of the brain called the pituitary gland.
Cortisol is normally released into the bloodstream at times of stress to prepare the body for a “fight or flight” response. In dogs with Cushing’s syndrome, cortisol is produced excessively all of the time and this eventually results in the development of the clinical signs of the syndrome.
There are 2 forms of hyperadrenocorticism
Pituitary-dependent hyperadrenocorticism (PDH) is the most common form (85% of cases) of the syndrome and occurs due to the development of a slow growing tumour in the pituitary gland. This tumour produces large amounts of the hormone ACTH.
Adrenal-dependent hyperadrenocorticism (ADH) occurs when a tumour producing large amounts of cortisol develops in one or possibly both of the adrenal glands.
Both tumours result in excessive amounts of cortisol in the blood and over time the clinical signs of Cushing’s syndrome will develop.