Spontaneous hyperadrenocorticism (HAC) is usually associated with either excessive secretion of adrenocorticotropic hormone (ACTH) from a pituitary tumor and subsequent cortisol production by the adrenal glands (pituitary dependent hyperadrenocorticism), or excessive secretion of cortisol from a functional adrenocortical tumor (adrenal dependent hyperadrenocorticism).

Testing for hyperadrenocorticism (HAC) is primarily indicated when there is a heightened suspicion due to clinical signs and other laboratory abnormalities typical of hyperadrenocorticism in an aged dog. Common clinical findings include polyuria, polydipsia, polyphagia, and panting. Physical exam findings may include a pot-bellied appearance, palpable hepatomegaly, and symmetrical and non-pruritic alopecia.

Increased liver enzyme activity, with alkaline phosphatase (ALP) disproportionately increased compared to alanine transaminase (ALT), is a frequent biochemical finding cited as a reason clinicians test for hyperadrenocorticism. However, ALP is a nonspecific enzyme, and increased ALP activity alone does not indicate the presence of hyperadrenocorticism or itself warrant testing for hyperadrenocorticism.

Testing for hyperadrenocorticism while a dog is systemically ill should be avoided, as nonadrenal illness can cause a false positive result on adrenal tests. Serum is the required sample type for measurement of cortisol.

Adrenal function test options include the low dose dexamethasone suppression test (LDDST) and adrenocorticotropic hormone (ACTH) stimulation test. The low-dose dexamethasone suppression test evaluates the pituitary and adrenal cortisol response to negative feedback from exogenous glucocorticoid. Dogs with hyperadrenocorticism are expected to show reduced sensitivity to the suppressive effects of administration of dexamethasone. The adrenocorticotropic hormone (ACTH) stimulation test evaluates adrenocortical reserve and tests for excessive production of cortisol in response to administration of ACTH.

Considerable debate exists around which test is “best” for diagnosis of naturally occurring hyperadrenocorticism as false negatives and false positives can occur with all adrenal tests. Consider the relative sensitivity and specificity of the low-dose dexamethasone suppression test and ACTH stimulation test when choosing a test and interpreting results. For instance, a negative low dose dexamethasone suppression test is more likely to be a true negative, since the low dose dexamethasone suppression test has higher sensitivity than specificity. Conversely, a positive ACTH stimulation test may offer more convincing support for hyperadrenocorticism since this test is more specific but less sensitive than the low dose dexamethasone suppression test.

When iatrogenic hyperadrenocorticism is a possibility, ACTH stimulation testing is the recommended test. A baseline or resting cortisol concentration (20017) alone cannot be used to diagnose or exclude hyperadrenocorticism as cortisol concentrations fluctuate throughout the day and in response to stress.

The urinary cortisol:creatinine ratio [UCCR] (20019) is a simple and noninvasive screening test for the presence of hyperadrenocorticism. A first-morning urine sample reflects the last several hours of cortisol production and minimizes the effect of cortisol fluctuation. Urine should be collected at home in a non-stressful environment. This test has high sensitivity but low specificity for hyperadrenocorticism. A urinary cortisol:creatinine ratio result within reference interval makes a diagnosis of hyperadrenocorticism unlikely; however, an elevated result does not confirm hyperadrenocorticism and an adrenal function test should be performed.

The ACTH stimulation test is recommended for identification of iatrogenic hyperadrenocorticism. Several factors should be considered before testing a dog for hyperadrenocorticism when exogenous glucocorticoids have been or are being given. First, glucocorticoids such as prednisone, prednisolone, and methylprednisolone cross-react in commercial cortisol assays. Second, the dog may have iatrogenic, rather than naturally occurring, hyperadrenocorticism.

Historical and clinical findings of hyperadrenocorticism can be seen with either endogenous cortisol production or exogenous administration of glucocorticoids by any route (injectable, oral, or topical preparations including dermal, ophthalmic, and otic). Chronic glucocorticoid administration can cause suppression of endogenous cortisol production and adrenal atrophy and, in this case, subnormal cortisol concentrations both pre- and post-ACTH stimulation can be the result.

Phenobarbital can cause clinical signs and biochemical changes resembling hyperadrenocorticism making it challenging to distinguish side effects of medication versus naturally occurring hyperadrenocorticism based on the clinical presentation. In humans and rats, phenobarbital has been shown to accelerate clearance of dexamethasone and contribute to false positive results on a low dose dexamethasone suppression test, but this effect has not been consistently demonstrated in dogs receiving phenobarbital. Phenobarbital has no known effect on ACTH stimulation test results.

Confirming a diagnosis of concurrent hyperadrenocorticism can be very difficult in a dog that has uncontrolled diabetes mellitus. Nonadrenal illness can give a false positive on any adrenal function test. Diabetes should be regulated before testing for concurrent hyperadrenocorticism.

Most dogs with hyperadrenocorticism have pituitary dependent hyperadrenocorticism (PDH) caused by a pituitary adenoma. Further testing to document either adrenal versus pituitary origin is perhaps most important prognostically and to determine if adrenalectomy could be an appropriate treatment option. Medical treatment with either trilostane (Vetoryl®) or mitotane (Lysodren®) can be effective in reducing the clinical signs of hyperadrenocorticism in dogs with either pituitary dependent or adrenal dependent hyperadrenocorticism.

After a diagnosis of hyperadrenocorticism has been made, but before treatment is initiated, measurement of endogenous ACTH (20006) may help distinguish between pituitary or adrenal hyperadrenocorticism. Dogs with an adrenal tumor often have a very low circulating concentration of endogenous ACTH. Conversely, a dog with pituitary dependent hyperadrenocorticism typically has an endogenous ACTH concentration that is within reference interval or increased. Unfortunately, some endogenous ACTH concentrations are inconclusive. Please see the recommendations in the test catalog for special handling instructions for preparation and shipment of samples for endogenous ACTH.

Monitoring therapy for hyperadrenocorticism should include both laboratory testing and assessment of the clinical response. The manufacturer of the brand name, Vetoryl®, recommends monitoring via an ACTH stimulation test performed four (4) to six (6) hours after administration of trilostane. The post-ACTH stimulation cortisol concentration is the most meaningful, with target ranges encompassing values unlikely to reflect over suppression but correlating with a good clinical response. Useful information for monitoring trilostane therapy can be found on the manufacturer’s website.

Recent attention has been given to baseline “pre-trilostane” cortisol concentration, without ACTH stimulation, for monitoring trilostane therapy. This approach has not been widely adopted in the United States, though this strategy has been utilized in Europe. A variety of suggested guidelines for using pre-pill cortisol in stable patients to monitor trilostane therapy have been published in the veterinary literature and on the manufacturer’s (United Kingdom) website.

A baseline cortisol concentration can be useful in excluding a diagnosis of hypoadrenocorticism if there is not a history of recent administration of a glucocorticoid which cross-reacts with the assay. Generally, baseline cortisol concentration greater than 55 nmol/L (greater than 2 μg/dL) makes a diagnosis of hypoadrenocorticism unlikely. A baseline cortisol alone cannot confidently diagnose hypoadrenocorticism, and an ACTH stimulation test is required to support the diagnosis. Both pre- and post-ACTH stimulation cortisol concentrations are expected to be very low or undetectable in a dog with untreated hypoadrenocorticism.

Exogenous corticosteroids can suppress the hypothalamic-pituitary-adrenal axis (HPAA), resulting in adrenal atrophy and subnormal pre- and post-ACTH stimulation cortisol concentrations. It can thus be difficult to confirm a diagnosis of hypoadrenocorticism following chronic administration of glucocorticoids (injectable, oral, or topical preparations including dermal, ophthalmic, and otic). Mineralocorticoid production is minimally affected by exogenous glucocorticoids. Therefore, aldosterone testing can be useful when cortisol results are confounded by recent steroid treatment.

Dexamethasone does not cross-react in the cortisol assay and will not cause a falsely increased cortisol concentration. Recent dexamethasone administration does inhibit endogenous cortisol production for a period of time. However, a single dose of dexamethasone given during the initial stabilization period of a suspected Addisonian crisis would be unlikely to result in both pre- and post-ACTH stimulation cortisol concentrations to be undetectable in a dog with normal adrenal function.

Measurement of aldosterone (20002) can be helpful in distinguishing primary from secondary hypoadrenocorticism and identifying dogs with selective glucocorticoid deficiency (atypical’ Addison’s). It should be noted that dogs with primary hypoadrenocorticism may still have electrolytes within reference intervals despite low or undetectable concentrations of both cortisol and aldosterone.

Measurement of endogenous ACTH may provide insight into the origin of hypoadrenocorticism. Primary hypoadrenocorticism characterized by bilateral adrenocortical destruction is the most common form in dogs, and the concentration of endogenous ACTH is expected to be increased in most dogs with untreated hypoadrenocorticism. Conversely, decreased endogenous ACTH is expected with secondary (central) hypoadrenocorticism.

Urinary cortisol:creatinine ratio (UCCR) can be informative when hypoadrenocorticism is suspected. Dogs with hypoadrenocorticism would be expected to have low urinary cortisol concentrations and a reduced calculated urinary cortisol:creatinine ratio.

Management of hypoadrenocorticism typically includes both glucocorticoid and mineralocorticoid replacement therapy. Most dogs have primary hypoadrenocorticism resulting from bilateral adrenocortical destruction, and endogenous production of cortisol and aldosterone is not expected to return. Thus, there is no benefit to measuring ACTH-stimulation cortisol and/or aldosterone concentrations once the diagnosis is established in a patient receiving both glucocorticoid and mineralocorticoid replacement therapy. Instead, the dog’s clinical signs and other biochemical testing (e.g., monitoring serum electrolyte balance), are utilized for evaluating response and making therapeutic decisions.