Addison’s and Cushing’s are both diseases of the adrenal glands. One involves over-production of hormones; the other under-production. So, they are essentially opposite conditions. Despite this, the actual tests performed to diagnose the conditions are the same. Let’s take a look at what tests are done for Cushing’s and Addison’s in Dogs. 

The Adrenal Glands 

The adrenal glands are a pair of glands that sit next to the kidneys and secrete several important hormones, including: 

  • Aldosterone (maintains sodium balance in the body) 
  • Cortisol (a steroid hormone, sometimes known as the ‘stress hormone’) 
  • Sex hormones  

The adrenal glands secrete cortisol when they receive a signal from the pituitary gland in the brain, via a chemical messenger called Adrenocorticotropic hormone (ACTH).  

What is Cushing’s Disease in Dogs? 

Cushing’s Disease is also known as Hyperadrenocorticism (HAC). It happens when the adrenal glands produce too much cortisol. There are two possible causes. It is most commonly caused by a tiny tumour in the pituitary gland, which is usually benign. This causes the adrenal glands to receive abnormal signalling from the pituitary gland, to produce too much cortisol. The other possible cause is a tumour in the adrenal glands themselves (this is less common). 

Cushing’s Disease in Dogs causes a wide range of symptoms, including: 

  • Drinking and weeing excessively 
  • A ‘pot-bellied’ appearance 
  • Increased hunger 
  • Panting excessively 
  • Lethargy, or low energy levels  
  • Slow healing wounds 
  • Bald patches on the flanks/ tail 
  • Muscle wastage 

If your dog is showing any of these signs, it’s time for a visit to the vets!  

It would be helpful if you could measure how much your dog drinks in 24 hours. You can do this using a measuring jug. Measure how much water you put in their bowl each time you top it up, and keep track. At the end of 24 hours, pour whatever is left in their bowl back into the measuring jug. Subtract this amount from the total volume you have put in their bowl over the 24 hour period. A fresh urine sample is also useful, if you can catch one!

What is Addison’s Disease in Dogs?

Addison’s Disease is the opposite of Cushing’s: it happens when the adrenal glands stop producing enough cortisol and aldosterone. In most cases, this is due to an autoimmune condition, where the dog’s own immune system starts attacking the adrenal glands by mistake.  

Early on in the disease, the signs of Addison’s are often vague and come and go. You may notice that your dog is just ‘not quite right’. Symptoms include: 

  • Drinking and weeing more than usual 
  • Lethargy 
  • Poor appetite 
  • Weight loss 
  • Sickness and diarrhoea that may come and go 

If you notice any of these signs, even if they are mild or they wax and wane, you should take your dog to a vet for a check-up. 

Addison’s is a life-threatening condition, which is fatal if left untreated. An ‘Addisonian Crisis’ can come on quickly; causing sickness, diarrhoea, collapse and a slow heart rate. If left untreated, this progresses to coma and, sadly, death. If you notice any of these symptoms, this is an emergency and you should contact your vet immediately. 

While Addison’s is relatively easy to diagnose, diagnosing Cushing’s is much more complicated!  

Your vet will start by examining your dog and asking you some questions. They may then advise routine blood and urine tests. These can be used to rule out other conditions with similar symptoms, and may offer some clues that are suggestive of Cushing’s or Addison’s. For example, in Cushing’s the urine will be very dilute, and the liver enzyme Alkaline Phosphatase (ALKP) is usually very high. In Addison’s, low sodium with high potassium is suspicious. 

Following on from there, your vet may suggest some specific tests to diagnose Cushing’s or Addison’s. 

Urine Cortisol to Creatinine Ratio (UCCR) 

The UCCR is a useful urine test to rule out Cushing’s in dogs. It is non-invasive, and the urine can be collected at home, where the stress of a vet visit cannot interfere with the results! If the UCCR is normal, we can be pretty certain that the dog does not have Cushing’s. However, if it comes back as high, it cannot be used alone to diagnose Cushing’s. This is because other conditions can also cause an increased UCCR.  

So, UCCR is a useful tool to rule out Cushings. If it comes back high, further tests will be needed to diagnose Cushing’s. 

Basal Cortisol 

Similarly to the UCCR for Cushing’s, a basal cortisol test can be used to rule out Addison’s, but not diagnose it. If the blood cortisol is above a certain level, your vet can rule out Addison’s without any further testing. If the blood cortisol is low, your vet will run further tests (an ACTH stimulation test) to see if your dog has Addison’s.

ACTH Stimulation Test 

This test can be used to diagnose both Addison’s and Cushing’s disease. The test involves taking a blood sample to measure cortisol levels, then injecting a synthetic ACTH, then a second blood sample for cortisol is taken 2 hours later. The test measures how the adrenal glands respond to ACTH. 

ACTH Stimulation Test for Addison’s  

In Addison’s, the ACTH stimulation test mimics the body’s natural stimulatory pathway (the pituitary gland releases ACTH, which stimulates the adrenal glands to produce cortisol). A healthy dog will show an increased cortisol after synthetic ACTH injection, since it stimulates the adrenals to produce cortisol. In Addison’s disease, the first blood sample will show low cortisol levels. Because the adrenal glands cannot produce cortisol, there will also be no response to the synthetic ACTH injection, so the second blood cortisol will only show a very tiny or no increase. This lack of response is diagnostic for Addison’s disease. 

ACTH Stimulation Test for Cushing’s  

In Cushing’s disease, the second cortisol sample is markedly or abnormally increased, as the adrenal glands respond excessively to the ACTH. Because the ACTH stimulation test will be positive for both types of Cushing’s disease (adrenal or pituitary origin), it cannot be used to tell which type a dog has.  

No one test is 100% accurate. However, false positives are not likely with this test. So, if the ACTH stimulation test is positive, it is very likely that your dog does indeed have Cushing’s. However, false negatives are possible, so your vet may wish to carry out further tests if their suspicions for Cushing’s are high. 

Low Dose Dexamethasone Suppression Test (LDDST) 

The Low Dose Dexamethasone Suppression Test can also be used to diagnose Cushing’s in dogs, as well as helping to differentiate between adrenal or pituitary causes.  

The LDDST is carried out by measuring blood cortisol, then injecting dexamethasone (a steroid that mimics cortisol) and taking further blood cortisol samples after 4 and 8 hours. In a dog without Cushing’s, the cortisol levels post injection would be lower than the first sample. This is because the dexamethasone signals the adrenal glands to stop producing cortisol. In dogs with Cushing’s, there will be no suppression. The 8 hour cortisol sample will be increased (higher than the normal reference range), since this ‘negative feedback’ will not work to stop the adrenal glands producing cortisol. 

False positives are possible with the LDDST, but if the result is negative, then it’s likely a true negative.  

The LDDST can also suggest which type of Cushing’s your dog has, and then your vet may advise imaging to confirm the suspicion.  

Final Thoughts 

Cushing’s and Addison’s are both diseases involving the adrenal glands, but they are opposites! Diagnosing Addison’s is quite straightforward, although it can be hard to spot initially since the symptoms are so vague. Diagnosing Cushing’s is more complicated and may require multiple or repeated testing for a definite diagnosis.  

If you notice any of the signs mentioned in your dog, seek veterinary advice as soon as possible.