Most pituitary tumours are benign (pituitary adenomas). They vary in size from small microtumours which hardly grow to large tumours which can compress or invade neighbouring tissues. In addition, pituitary tumours can excessively secrete hormones.
In the adenohypophysis, several different hormones are produced and secreted. Pituitary tumours can be either functional (actively secreting hormones) or non-functional (no hormone secretion).
In dogs, the most commonly diagnosed pituitary tumour type is an adrenocorticotropic hormone (ACTH)-secreting tumour. This excessively secreted ACTH will signal to the adrenal glands that they have to produce more cortisol.
This uncontrolled and excessive cortisol production is known as hypercortisolism or Cushing’s syndrome. In most cases (~75%) hypercortisolism is caused by pituitary tumours, which is referred to as pituitary-dependent hypercortisolism (PDH. In 25% of cases hypercortisolism is caused by adrenal tumours or adrenal dysfunction, which is referred to as adrenal-dependent hypercortisolism (ADH) – more about this via adrenal cortex tumour.
Another type of functional pituitary tumours are growth-hormone secreting tumours, but these are far less common.
The prevalence of PDH is approximately 1 in every 500 dogs, and mostly occurs in middle-aged and older dogs. The prevalence of (small) non-functional pituitary tumours can be underestimated, because they do not cause clinical symptoms.
A higher risk to develop Cushing’s syndrome has been reported for the following breeds: basset hound, bichon frisé, border terrier, dachshund, German shepherd, Irish setter, miniature schnauzer, miniature poodle, and Yorkshire terrier.
The onset of hypercortisolism is often slow, and the signs can progress slowly. The most common clinical signs in dogs with PDH include excessive eating and drinking, increased urination, muscle weakness, accumulation of fat in the abdomen, panting, progressive bilateral alopecia, thinning of the skin, and poor wound healing.
When the pituitary tumour is large, it can cause pressure on neighbouring brain tissue and induce neurological signs such as anorexia, lethargy, and altered behaviour.