What are the missing hormones in Addison's disease?
Dogs with hypoadrenocorticism (Addison's disease) are unable to produce one or two steroid hormones, both secreted from the adrenal cortex (outer layer of the adrenal gland).
- The first hormone that's missing is cortisol, which is very important in maintaining a normal metabolism, as well as a general sense of well being.
- The second hormone that's missing is aldosterone, which manages the water balance and serum electrolytes in the body.
Hypoadrenocorticism can be divided into primary and secondary subtypes. A third form, called "atypical" Addison's disease appears to be an early stage of primary hypoadrenocorticism in most dogs.
- With primary hypoadrenocorticism (Addison's disease), the problem lies in the adrenal gland itself, with atrophy or destruction of the gland.
- With secondary hypoadrenocorticism, the adrenal gland are normal, and the problem lies in the pituitary gland. The pituitary gland normally secretes a hormone called ACTH (adrenocorticotropic hormone) that stimulates the adrenal gland to secrete its hormones; in secondary hypoadrenocorticism, ACTH is not secreted in needed amounts, leading to the secondary adrenal insufficiency.
- The third subtype, atypical Addison's disease appears to be an early stage of primary hypoadrenocorticism in which only part of the adrenal gland has been completely destroyed.
In most dogs, we cannot determine what caused their adrenal disease. With primary hypoadrenocorticism, the following causes must be considered.
- Most dogs with the primary form appear to have an autoimmune condition in which the body destroys part of the adrenal cortex
- Very rarely, infiltrative conditions such as cancer can metastasis to and destroy the adrenal gland to cause Addison’s disease
- Occasionally, treatment of Cushing’s disease with the drugs mitotane and trilostane will result in complete adrenal destruction and Addison’s disease
- Congenital deficiency of ACTH secretion
- Pituitary tumors, inflammation or trauma that have destroyed most of the ACTH-secreting cells in the pituitary gland, leading to deficient ACTH secretion
- The mechanism by which atypical Addison's disease develops in not well-understood
- Most of these dogs appear to have an early stage of primary hypoadrenocorticism
- In these dogs, destruction of the adrenal cortex is not complete, making diagnosis more difficult
Although hypoadrenocorticism can develop in any age, breed, or sex of dogs, the following points should be taken into consideration when making a diagnosis.
- Hypoadrenocorticism is most common in young to middle-aged dogs.
- Most dogs with Addison's disease are females.
- Bearded Collies, Standard Poodles (see photo on left, below) , Great Danes, Portuguese Water Dogs, West Highland White Terriers (see photo on right, below), and Leonbergers are all predisposed to developing this disease.
With primary hypoadrenocorticism (Addison's disease), the dog generally develops complete adrenocortical destruction with both cortisol and aldosterone deficiencies.
Aldosterone is the main mineralocorticoid hormone, and it affects the levels of potassium, sodium, and chloride in the blood. Low levels of aldosterone cause potassium to gradually build up in the blood and, in severe cases, cause the heart to slow down or beat irregularly. Some dogs have such a slow heart rate (50 beats per minute or lower) that they can become weak or go into shock.
Less commonly, dogs will develop secondary or "atypical” primary hypoadrenocorticism. This subgroup of dogs are only deficient in cortisol and appear to maintain a normal aldosterone level, at least early in the course of their disease. These dogs are more difficult to diagnose because their signs are milder and the serum potassium, sodium, and chloride values all remain normal.
Clinical features of hypoadrenocorticism in dogs
Historically, hypoadrenocorticism generally has a waxing and waning course and may be confused with other diseases because the clinical signs are not specific. The most common clinical signs and physical exam finding include the following:
- Depression or lethargy
- Weakness or collapse
- Anorexia (poor appetite)
- Weight loss
- Vomiting or diarrhea
- Excessive thirst and urination
- Hypothermia (low body temperature)
- Dehydration and shock
In my next post, I'll discuss how we can diagnose hypoadrenocorticism, can will be fatal if not properly managed.