Addison’s disease, otherwise known as hypoadrenocorticism, is an endocrine abnormality that affects dogs and, rarely, cats. It can be a bit tricky to understand, especially because there are various types. Essentially, Addison’s occurs because of a lack of functional tissue in the adrenal gland; reduced hormone production in the adrenal gland leads to impaired water balance and affects how well the animal can respond to stressful insults, such as illness. 

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What is the adrenal gland?

The dog has two adrenal glands, they are found inside the abdomen and there’s one just in front of each kidney. There is an inner medulla, and an outer cortex; in hypoadrenocorticism, it’s the outer cortex that is affected. The pituitary gland in the brain produces a substance called adrenocorticotropic hormone (that’s quite a mouthful so we call it ACTH!). This ACTH stimulates the adrenal gland to release hormones. 

What happens in Addison’s?

The hormones made by the adrenal gland include cortisol and aldosterone. Cortisol is important in immune and stress responses, gastrointestinal health and many other roles, whilst aldosterone is key in controlling electrolyte (salt) and water balance, which helps regulate the body’s hydration level. 

The types of hypoadrenocorticism we see in pets:

Classical hypoadrenocorticism

The destruction of the outer cortex layers by the body’s own immune system causes abnormally low levels of cortisol and aldosterone. Sometimes we catch this when only the cortisol levels are low making it look like hypocortisolism and not typical Addison’s.

Secondary hypoadrenocorticism

Lack of ACTH from the pituitary gland leads to lack of release of cortisol and aldosterone from the adrenal glands. This can occur when patients have been taking steroid medication long-term but it is abruptly stopped. The steroids act on the pituitary to prevent ACTH secretion so the levels of ACTH are low. This means that the adrenal gland becomes suppressed and doesn’t have the reserves to respond as necessary immediately after the steroids are stopped.

This is why we always say to taper doses of long-term steroid medications gradually. It allows the ACTH levels to increase and the adrenal glands to regenerate. Rarely pituitary problems lead to low ACTH levels and secondary hypoadrenocorticism.

Hypocortisolism

Sometimes called atypical hypoadrenocorticism. Here only the part of the adrenal gland that produces cortisol is affected so dogs don’t have problems with water balance. 

What are the signs of Addison’s and how is it diagnosed?

Hypoadrenocorticism, in the vet world, is nicknamed ‘the great pretender’. This is because its signs can be quite vague, making it appear like a whole bunch of other illnesses. As a result t dogs may go undiagnosed for a while.

The typical history of a dog with Addison’s is on and off gastrointestinal problems (vomiting, diarrhoea and inappetence) for a few weeks to months. These episodes get better with basic treatment, but they keep happening. After a while, the dog presents as an emergency, collapsed and showing signs of shock – often associated with a stressful event – called an Addisonian Crisis. Other vague signs include lethargy, bloody stools, abdominal pain and disorientation. 

Once in a Crisis, these pets are critical and need help to restore their fluid balance. They have abnormally slow heart rates because of electrolyte disturbances, requiring intensive fluid therapy to correct. 

How will the vet check your dog?

The veterinarian will take bloods which often show an abnormally normal haematology (normally we see typical changes in shocky pets but these aren’t present, or are reversed in Addison’s patients). On biochemistry, we often see changes similar to kidney failure, but the key is in the electrolytes: the potassium is higher than normal and the sodium lower than normal.

Sometimes imaging studies like X-rays or ultrasound are done to rule out other diseases and check the size of the adrenal glands. But diagnosis relies on demonstrating abnormal responsiveness of the adrenal glands by measuring the cortisol levels before and after the injection of ACTH. If the levels are low before and after ACTH then the adrenal glands aren’t responding normally and hypoadrenocorticism is diagnosed. 

What does Addison’s mean for my dog, can it be treated?

The good news is that dogs with hypoadrenocorticism can live generally normal lives, but they do need life-long medication. In the beginning, it can take some time and repeated testing to get electrolyte levels under control and get affected dogs on the correct doses.

Previously we used mostly tablet medication to help stabilise the electrolyte and cortisol levels. However, good control can now be achieved by giving daily prednisolone tablets and approximately monthly injections of DOCP (desoxycortone pivalate). This is used to fulfil the action of aldosterone. It can take a little while and repeated blood tests in the beginning, in order to get them on the right dose and dose interval or DOCP, so it’s important to be patient. 

Dogs with Addison’s disease need a bit of upkeep, so it’s important not to miss doses of medication. However, most can enjoy a good quality of life. 

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