How to Read Cortisol Test Results

WeightQ&AMeongsiljang Veterinary Advisory Board

We’ve put together an easy-to-understand guide for pet owners on the types of cortisol tests for dogs and cats, normal reference ranges, what high or low results may indicate, and recommended follow-up tests.

What Is a Cortisol Test?

A veterinarian drawing blood from a puppy.

A cortisol test is a blood test that measures the level of this stress hormone produced by the adrenal glands to help identify hormonal disorders such as Cushing’s disease and Addison’s disease. What’s truly important is that we don’t rely on a single blood cortisol level alone. Cortisol levels can fluctuate significantly depending on stress and time of day, so functional tests like the ACTH stimulation test are necessary for an accurate diagnosis. Please don’t jump to conclusions about whether your pet has a disease or not based on just one number on the results sheet.

When Should You Get Tested?

Cortisol-related hormonal disorders often develop gradually, making their symptoms easy to mistake for normal aging. If you notice any of the following signs persisting for more than two weeks, consider having your pet evaluated. - Polyuria and polydipsia: Drinking significantly more water than usual and producing larger volumes of urine - Increased appetite: Continuing to act hungry even after finishing meals - Abdominal distension: A protruding, sagging belly accompanied by muscle wasting - Hair loss and thinning skin: Symmetrical hair loss on both sides of the body with noticeably thinner skin - Lethargy and vomiting: In cases of Addison’s disease, sudden dehydration and shock can occur

Comparison of Cortisol Test Types

Item	Basal cortisol	ACTH stimulation test	Low-dose dexamethasone suppression test
Purpose	Screening	Addison's confirmation · Cushing's screening	Cushing's screening
Duration	15 minutes	1–2 hours	8 hours
Number of blood draws	1 time	2 times	3 times
Accuracy	Low	High	Very high
Fasting required	Not required	Required	Required

Your veterinarian selects the test based on the suspected condition and your pet's status

How to Interpret Normal Reference Ranges

Cortisol levels can vary slightly depending on the laboratory and the units used, so it’s important to review the "reference range" listed on your results sheet. Here’s how to interpret the general guidelines: - Basal cortisol: A level roughly between 2 and 6 μg/dL is considered within the normal range. - After ACTH stimulation: A level typically between 6 and 18 μg/dL is normal. If the result exceeds the upper limit of the reference range (18 μg/dL), Cushing’s disease is suspected. If it remains below the lower limit of the reference range (6 μg/dL) even after stimulation, Addison’s disease is suspected. - After dexamethasone administration: Blood is drawn 4 and 8 hours later to confirm that cortisol levels have been sufficiently suppressed below the normal reference range. According to veterinary internal medicine textbooks, the key to diagnosis lies not in a single value, but in the "pattern of change before and after the test."

A scene showing the review of cortisol test results.

Important Considerations for Interpreting Results

Do not rely on a single cortisol test to confirm a diagnosis. The stress of a vet visit alone can significantly alter baseline cortisol levels. Additionally, recent use of steroid medications—whether topical creams, inhalers, eye drops, or oral drugs—can be absorbed into the body and suppress the hypothalamic-pituitary-adrenal (HPA) axis, leading to distorted adrenal function test results. These changes caused by external steroids will only normalize once the medication is gradually tapered off, allowing the HPA axis to recover. Therefore, it is essential to inform your veterinarian about all medications used recently, including ear, skin, and eye treatments, to ensure accurate interpretation of the test results.

What Does a High Result Mean?

If cortisol levels rise excessively above the normal reference range upper limit (18 μg/dL) after ACTH stimulation, we suspect Cushing’s disease (hyperadrenocorticism). Cushing’s disease is broadly categorized into two types based on the underlying cause: - Pituitary-dependent type: A pituitary tumor overproduces ACTH, causing both adrenal glands to enlarge. - Adrenal tumor type: A tumor develops in one adrenal gland, leading to uncontrolled cortisol production. However, the ACTH stimulation test alone cannot distinguish between these two types. Because treatment options, including medication and the need for surgery, differ significantly depending on the type, further diagnostic tests such as a high-dose dexamethasone suppression test (which can identify the cause in approximately 75% of dogs) or an abdominal ultrasound are necessary to pinpoint the source.

Ultrasound examination of a dog's adrenal glands

What Does a Low Result Mean?

If cortisol levels fail to rise above the lower limit of the normal reference range (6 μg/dL) during an ACTH stimulation test, Addison’s disease (hypoadrenocorticism) should be suspected. Because Addison’s disease commonly presents with nonspecific symptoms such as vomiting, diarrhea, and lethargy, it is often mistaken for gastrointestinal illness. When accompanied by electrolyte imbalances—specifically hyperkalemia and hyponatremia—it can progress to an Addisonian crisis, a life-threatening emergency requiring immediate veterinary care with intravenous fluids and steroid therapy.

When Test Results Are in the Borderline Range

If test results fall in the borderline range between normal and abnormal, we recommend retesting in 4 to 8 weeks or conducting additional functional tests. Don’t worry over a single “equivocal” value; instead, work with your veterinarian to make a diagnosis based on a combination of clinical signs, imaging studies, and follow-up test results. Starting treatment prematurely can actually disrupt normal adrenal function.

Pre-Test Preparation and Follow-Up Care

To ensure accurate test results, please follow these guidelines: - Fasting: Withhold food for 8–12 hours before the test, but water is allowed. - Medication Review: Provide your veterinarian with a list of any steroids, anticonvulsants, or immunosuppressants your pet has taken in the past month. - Minimize Stress: Bring your pet’s familiar blanket or toys on the day of the test to help reduce stress-induced fluctuations in the results. After diagnosis, follow-up tests should be conducted every 3–6 months to adjust medication dosages as needed. For more detailed information on Cushing’s disease treatment, please refer to our Cushing’s Disease Management Guide.

Reviewed by a veterinarian

Dr. Tony — Punnawat Phongkittirak

Veterinarian

A veterinarian who majored in veterinary medicine at Khon Kaen University, Thailand, and completed the IVSA program at North Carolina State University in the United States. Drawing on clinical experience at animal hospitals, he works in the pet healthcare field and is dedicated to building a digital care environment that connects pet parents with veterinarians.

Frequently Asked Questions

How much does a cortisol test cost?

While costs vary by clinic, a baseline cortisol test is relatively affordable, whereas an ACTH stimulation test tends to be more expensive due to the cost of the medication involved. For exact pricing, please contact your visiting clinic directly.

Will the test results be available on the same day?

If the clinic has in-house diagnostic equipment, results are typically available within 1 to 2 hours. However, if samples need to be sent to an external laboratory, it may take 2 to 3 days.

Do cats undergo the same type of testing?

The underlying principle is the same, but Cushing’s disease is rare in cats and often occurs alongside diabetes. The normal reference ranges and interpretation criteria also differ slightly from those for dogs.

Does a normal test result mean I can be completely reassured?

If symptoms persist, it may indicate an early stage of the condition, so consider retesting in 4 to 8 weeks. Hormonal diseases progress gradually, so a single normal result does not guarantee lifelong safety.

I'm worried that the test results might be inaccurate due to stress from being at the clinic.

While baseline cortisol levels can be influenced by stress, the ACTH stimulation test is much less affected because it measures the change in cortisol before and after administration. This makes it a more reliable diagnostic test.

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References

[1] Feldman EC, Nelson RW, Reusch C, Scott-Moncrieff JC. Canine and Feline Endocrinology, 4th Edition, Elsevier Saunders, 2015

[2] Ettinger SJ, Feldman EC, Cote E. Textbook of Veterinary Internal Medicine, 8th Edition, Elsevier, 2017

[3] Behrend EN et al., Diagnosis of spontaneous canine hyperadrenocorticism: 2012 ACVIM Consensus Statement, Journal of Veterinary Internal Medicine, 2013