NAVLE Nuggets: Cushing's vs. Addison's Disease (LDDS vs. ACTH Testing)

Case number one.

An eight-year-old miniature poodle is presented for several months of increased thirst and urination, increased appetite, panting, a pot-bellied appearance, muscle weakness, and bilateral truncal alopecia.

Blood work shows an increased ALP and hypercholesterolemia.

What is your top differential? The answer is hyperadrenocorticism or Cushing's disease.

Hyperadrenocorticism means the dog has too much cortisol.

The classic clinical signs, just like we saw in this case, are PUPD, polyphagia, panting, potbelly, and muscle weakness, and sometimes alopecia as well.

How do we remember this? We wanna think Cushing's equals cortisol is crushing everything, or simply high cortisol equals high everything.

These animals are hungry, thirsty, peeing, and panting, and they're also losing muscle and hair.

So how do we diagnose Cushing's? First, we wanna make sure that our clinical signs are in line with hyperadrenocorticism.

What's our next step? We want to perform an LDDS or low dose dexamethasone suppression test.

The low dose dexamethasone suppression test is the screening test of choice for canine hyperadrenocorticism because of its high sensitivity.

High sensitivity means low amount of false negatives.

We can remember this by remembering sensitivity equals snout, which is, this is a highly sensitive test, meaning we can trust a negative and negatives rule out the disease.

So snout, highly sensitive, means we trust a negative and we can use a negative to rule a disease out.

Now back to our LDDS and how it works.

Here's the concept.

Normally, dexamethasone acts like cortisol and tells the pituitary, we have enough cortisol, stop making ACTH.

So in a normal dog, we would expect that dexamethasone decreases ACTH and decreases cortisol through negative feedback.

But in a dog with Cushing's, that negative feedback loop doesn't work normally.

So cortisol fails to suppress appropriately.

The typical LDDS protocol measures cortisol before dexamethasone, four hours after, and eight hours after.

In a dog with hyperadrenocorticism or Cushing's disease, an eight-hour cortisol concentration above the laboratory reference interval supports the diagnosis.

Our Navly memory trick to remember Cushing's is that Cushing's can't suppress.

If cortisol does not suppress after dexamethasone, we want to think about diagnosing Cushing's disease.

Now here's where things get tricky.