Primary hypoadrenocorticism (or classical Addison's disease) is a relatively uncommon but important disease of dogs. Primary hypoadrenocorticism results from destruction of the adrenal gland leading to deficiency of both minerocorticoids and glucocorticoids. The clinical signs can be non-specific, and while the first clue that this disease is present is often an alteration in the sodium:potassium ratio, some cases may be "atypical" in that hyperkalaemia and hyponatraemia are not always present.
Other clinicopathological findings that may be present include hypoglycaemia, hypercalcaemia, hyperphosphataemia, mild nonregenerative anaemia, lymphocytosis, eosinophilia, prerenal azotaemia and dilute urine.
ACTH stimulation test
The diagnosis of hypoadrenocorticism is confirmed with an ACTH stimulation test.
- In hypoadrenocorticism, baseline cortisol levels are usually <28 nmol/L and post-ACTH values are <28nmol/L.
As the electrolyte changes often present in these dogs can be life threatening and the stimulation test must be carried out before any treatment can be given, it is advisable to have Synacthen® available at all times in the clinic. The ACTH stimulation test should ideally be performed before administration of therapeutic corticosteroids. When rescue dexamethasone is given prior to diagnostic testing, the ACTH stimulation test should ideally be completed within 3 hours to avoid potential confounding of results by corticosteroid-mediated suppression of ACTH, which may lead to adrenal atrophy within several hours.
Secondary hypoadrenocorticism occurs when there is a lack of ACTH secretion by the pituitary gland, resulting in low serum cortisol concentrations, with clinical signs related to low cortisol rather than electrolyte disturbances. Electrolyte concentrations will be normal because aldosterone secretion is usually not affected. An ACTH stimulation test will show a low baseline serum cortisol concentration and minimal response to ACTH.
Hypoadrenocorticism is a rare disease in cats. The same ACTH protocol is recommended.