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ENDOCRINOLOGY: HYPOTHYROIDISM IN DOGS

YVCipedia ENDOCRINOLOGY
HYPERADRENOCORTICISM (HAC) (AKA CUSHING'S DISEASE) IN DOGS

HAC is a syndrome caused by abnormally high levels of adrenal hormones. These natural hormones are produced by the adrenal glands, a paired set of glands located, one near each kidney, in the abdomen of most mammals. The production of hormones by the adrenal glands is closely regulated by the pituitary gland, a small gland at the base of the brain. 

Abnormally increased production of hormones may be caused by a tumor in one or both adrenal glands, or a tumor in the pituitary gland. A similar syndrome is iatrogenic HAC, which is caused by administration of medications that contain cortisone. (When cortisone medications are used at the common doses and on the common schedules the risk of iatrogenic HAC is extremely small.)

SIGNS
The signs of HAC are extremely variable - pets most commonly have one or a few signs and not many or all signs -  and often subtle. They include:

No signs (HAC discovered by chance when testing for other problems)
Increased thirst and urination
Loss of muscle mass and muscle tone; abdominal muscles can weaken causing a pot-belly
Thin hair coat
Skin blackheads
Hard, flat, painful nodules in the skin
Increased skin pigmentation
Shrinkage of the testicles
Failure of the heat cycle
Obesity
Panting
Paralysis of facial muscles

Because HAC typically occurs in middle-aged to older pets owners sometimes attribute their pet's signs to normal aging. Successful treatment for HAC will result in partial to complete resolution of signs, showing that the problem was not aging at all. 

DIAGNOSIS
Because the signs are usually subtle, and because none of the signs are specific for HAC (that is, the same signs can occur with a number of other problems), the process of diagnosing HAC is often not straightforward. 

Once we are relatively certain a patient has HAC, additional testing is often needed to determine if  it is being caused by a tumor in the pituitary gland, or a tumor in the adrenal glands. Distinguishing between these two forms of HAC is necessary for us to recommend appropriate treatment and give an accurate prognosis.

There is no single "yes-or-no" test for HAC.  The tests most commonly used are:

General health screening
    CBC, general blood profile
    Urinalysis
    X-rays of the chest and abdomen

HAC screening
    Urine cortisol/creatinine ratio

HAC confirmation
    ACTH stimulation blood test
    Abdominal ultrasound

Occasionally, advanced testing
    CT scan
    MRI

TREATMENT OPTIONS
Just as there is no "yes-or-no" test for diagnosing HAC, there is no simple curative treatment for it either.

The pituitary form of HAC is treated with medication. There are a few different options; the safest and most effective option currently is trilostane. Trilostane suppresses the overproduction of hormones by the adrenal glands; it is not chemotherapy intended to destroy the pituitary tumor. Because of this mode of action, treatment must be continued for the life of the pet. For a small number of pets with HAC trilostane will not work well, or at all.

Each pet's trilostane dose must initially be adjusted to lower the adrenal hormones to a normal level, but not below a normal level; serious problems can occur if these hormone levels become too low. Finding this appropriate dose usually involves repeating the ACTH stimulation blood test one to four times over several weeks. 

The dose of trilostane a pet needs can change over time, so the adrenal hormone levels must be regularly monitored. We usually accomplish this by repeating the ACTH blood test once every six months.

The adrenal tumor form of HAC can sometimes be treated with surgery. Even in the best of circumstances removing an adrenal tumor is a very challenging procedure. Also, when there is a tumor in one adrenal gland, the opposite adrenal gland shrinks and becomes inactive, and nursing the patient through the immediate post-operative procedure can be very difficult as well. Some adrenal tumors are inoperable. For patients with adrenal tumors that will not undergo surgery we can attempt medical treatment with trilostane but this is not as likely to work as well as it can for the pituitary form of HAC. 

One other treatment option is no treatment. HAC, especially the pituitary form, is typically a problem that slowly progresses over months to years and is not painful. The signs are often low-grade and managable by the pet parent; for example, a pet with increased thirst and urination can be taken out more often during the day and even have its water consumption very carefully limited. Thus, the no-treatment approach is sometimes the best approach. 

Yarmouth Veterinary Center
2013, updated 2015

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