Diagnosing canine hyperadrenocorticism: You have questions, Dr. Endocrine has answers

Does diagnosing canine hyperadrenocorticism make your head tilt? "Dr. Endocrine" can help. (Getty Images)Some weeks it seems like every single low-dose dexamethasone suppression test (LDDS) or adrenocorticotropin hormone (ACTH) stimulation test you perform yields ambiguous results. ARGH! So, what's a diagnostician like you to do (besides stay awake all night and agonize over the outcomes)? Keep calm, and read on.

When your mind races, go back to the basics

Cushing's syndrome involves a constellation of clinical signs attributable to an elevated cortisol concentration in the blood, which can be caused by hyperadrenocorticism.

There are two main forms of hyperadrenocorticism (note: there are a few rare others, but for the sake of sanity and time, we will skip over those for now). Upwards of 85% of your patients will have pituitary-dependent hyperadrenocorticism (PDH) from a pituitary tumor. The remaining 10 to 12% will have adrenal-dependent hyperadrenocorticism, which is caused by a cortisol-secreting unilateral or bilateral adrenal tumor.

Question #1: Where do I start?

Perform a thorough physical examination, including a complete blood count, a serum chemistry profile and a urinalysis. Request a urine culture and sensitivity testing, as well. About 30% to 40% of dogs with hyperadrenocorticism have urinary tract infections-even with no clinical signs or evidence of infection on the urinalysis-most likely because hypercortisolemia is immunosuppressive and causes dogs to become polyuric.1-3

Question #2: Does it really matter what time of day I start and stop drawing blood when testing for hyperadrenocorticism?

No. Unlike people, dogs and cats secrete ACTH (resulting in increased cortisol production) episodically-about every 20 to 30 minutes. Thus, cortisol concentrations in the blood don't vary much throughout the day, so testing should be equivalent and acceptable at any time. Truly erroneous test results tend to occur in older dogs with concurrent diseases that are receiving medications that interfere with adrenal function test results. Medications like phenobarbital and ketoconazole can significantly alter adrenal function tests.

Question #3: What do I do when I'm positive the miniature poodle in front of me is cushingoid, but the results come back normal?

As an attendee at one of my lectures once answered, “You keep going until you get the answer that you like.” So, go ahead-run another test.

There are three common tests for measuring cortisol, including a urine cortisol-to- creatinine ratio test and two types of blood tests: an LDDS test and an ACTH stimulation test-all of which can give false positive and negative results. And because they all measure different things, if you get discordant results, it doesn't necessarily mean that one of the tests is wrong. Results can vary based on the phase of hyperadrenocorticism the dog is currently experiencing. Dogs with early PDH may not have developed adrenal hyperplasia yet, so an ACTH stimulation test may be normal and small pituitary tumors may still respond to dexamethasone.

3 more helpful tips

1.) If you do an ACTH stimulation test on a dog with iatrogenic hyperadrenocorticism, the results will appear as if the dog is addisonian. The pre- and post-cortisol concentrations will be low and low.

2.) Otic or ophthalmic corticosteroid products, when given to small dogs (10 kg or less), can decrease or flatten their ACTH stimulation test results.

3.) A small dog with no history of corticosteroid administration that looks cushingoid but has flat ACTH stimulation test results may be affected by its owner. Ask the client if anyone in the house is using hormone replacement therapy gel, as it can be transferred to the pet with even minimal contact.

Question #4: What do the tests measure?

ACTH stimulation test

Does this dog have increased adrenal mass?

The ACTH stimulation test protocol and reasoning behind it are fairly simple, so I'm going to mix things up and ask you a question: Which kind of cosyntropin do you use: compounded gel or injectable pre-prepared product?

Interestingly, researchers compared the two formulations in normal and cushingoid dogs and couldn't tell a difference.4 However, they also analyzed the concentrations of cosyntropin in the gel preparations and found they varied widely. These findings tell us that the dose of cosyntropin dogs need for maximum adrenal stimulation is tiny-possibly as low as 1 µg/kg-not an entire 250-mg vial per dog.

Some experts (including myself) currently recommend using injectable cosyntropin at a dose of 5 µg/kg. I know what you are thinking: “But wait, Dr. Cushing … er-Dr. Bruyette! The thought of tossing the extra cosyntropin left in the open vial makes my head (and wallet) hurt!” Fear not. Instead, following reconstitution, use the amount you need for the present test, and leave the rest in the vial. If you think you will use the leftovers within the next 30 days, put the vial in the refrigerator. If not, draw it up into 0.1-ml aliquots in plastic syringes and store in the freezer where it will stay good for at least six months.5

LDDS test

Is there loss of feedback between the pituitary gland and the adrenal glands?

The LDDS test is not an adrenal function test. It is a pituitary function test. The classic hallmark of PDH is a lack of communication between the pituitary gland and the adrenal glands. The pituitary does not “hear” the adrenal glands' feedback. The LDDS test can diagnose PDH in about 90% of dogs with PDH and allows you to determine both whether a not a dog has hyperadrenocorticism and if it is pituitary-dependent.

If dexamethasone is administered at the test dosage to a normal dog, it will be suppressed for about 24 hours. If the dog has been receiving iatrogenic corticosteroids, LDDS test results will be completely unreliable because the dog is already chronically suppressed, and you will have to do an ACTH stimulation test.

If you have diagnosed hyperadrenocorticism by performing an LDDS test, you do not need to perform an ACTH stimulation test to establish a baseline set of cortisol concentrations. It doesn't really matter what the cortisol concentrations were to start. We should just care that they are less than 9 µl/dl after the patient begins treatment.

Want to know the easiest and fastest way to interpret LDDS test results? Go straight to the eight-hour measurement of cortisol concentration. Here are two more pearls of wisdom:

1.) If the eight-hour measurement of cortisol concentration is elevated, the dog has hyperadrenocorticism. Now you care about the other measurements of cortisol concentration on the results chart.

2.) If there is at least 50% suppression of cortisol concentration at any time point in the study, the animal must have PDH. Period. But here's the kicker: If you don't see less than or equal to 50% suppression at any time during the test, you can't rule out PDH.

Urinary cortisol:creatinine ratio

Does this dog produce more cortisol in a 24-hour period than a normal dog?

This test has the highest sensitivity of the three but has poor specificity. Positive results must be confirmed with an LDDS test.

To drive down the false positive rate, follow proper protocol for sample collection. The first morning urine should be obtained by the owner at home three days in a row. The three urine samples should then be pooled together by the owner and brought into the clinic for testing.

Now what?

I hope my tips help you rest a little easier at night. Want to sleep even better? Read what I have to say about treating hyperadrenocorticism-coming up next!

References

1. Hess RS, Saunders HM, Van Winkle TJ, Ward CR. Concurrent disorders in dogs with diabetes mellitus: 221 cases (1993-1998). J Am Vet Med Assoc 2000;217(8):1166-1173.

2. Forrester SD, Troy GC, Dalton MN, et al. Retrospective evaluation of urinary tract infection in 42 dogs with hyperadrenocorticism or diabetes mellitus or both. J Vet Intern Med 1999;13(6):557-560.

3. Ling GV, Stabenfeldt GH, Comer KM, et al. Canine hyperadrenocorticism: pretreatment clinical and laboratory evaluation of 117 cases. J Am Vet Med Assoc 1979;174(11):1211-1215.

4. Kemppainen RJ, Behrend EN, Busch KA. Use of compounded adrenocorticotropic hormone (ACTH) for adrenal function testing in dogs. J Am Anim Hosp Assoc 2005;41(6):368-372.

5. Frank LA, Oliver JW. Comparison of serum cortisol concentrations in clinically normal dogs after administration of freshly reconstituted versus reconstituted and stored frozen cosyntropin. J Am Vet Med Assoc 1998;212(10):1569-1571.