May 17, 2012

Canine Wellness
September 2007

Canine Hypothyroidism
by Dave Erlewein, D.V.M.

ypothyroidism, or abnormally low levels of the thyroid hormone, is a common affliction for pointing dogs and retrievers alike. In this article, we’ll explore how it occurs, how it affects your dog, and how it’s diagnosed and treated.

The thyroid gland is located in the neck, just below and behind the larynx, and is shaped like the letter H, with lobes on both the right and left side of the windpipe. The gland produces two forms of the thyroid hormone, T4 and T3. T3 is the active form of the hormone utilized by the body tissues, while T4 circulates in the blood, bound or unbound (free T4) to proteins. Free T4 is readily converted to the tissue-active form T3. Thyroid hormone acts like a throttle for the body’s metabolic rate and influences many of the body systems. Elevated levels speed up metabolism and reduced levels slow it. 

When the pituitary gland, frequently called the master gland, senses the levels of T4 in the blood are low, it produces Thyroid Stimulating Hormone (TSH), which does exactly what the name says -- stimulates the thyroid gland to produce T4 and T3.

Hypothyroidism can be present at birth (congenital) or brought on by a dietary deficiency of iodine, but the primary causes in dogs are the result of either a dysfunction of the immune system that destroys the gland, or a simple atrophy or “wearing out” of the gland. . Though it usually appears in middle-aged and elderly dogs, it may appear at any age, in any dog, purebred or mixed.

he most common signs of hypothyroidism are changes in appearance, weight, and activity. More than three-quarters of affected dogs will exhibit skin and coat changes, such as excessive shedding, premature graying of the hair on the muzzle and around the eyes, patches of hair loss, scaly or flaky skin, brittle or dry coat, skin infections, and unpleasant coat odor. Occasionally there will be severe hair loss on the tail, a condition known as “rat tail.”

Nearly half of the affected dogs will be obese or show a sudden increase in weight and appear listless or lethargic unless there is some unusual stimulus or activity.

Other less common problems include anemia (low red blood cell count), slow heart rate, abnormal heart rhythm, and occasionally afflictions of the central nervous system such as problems with gait and balance, and head tilt.

The most important thing to remember about symptoms is that an individual dog may have several symptoms or only one. Though the classic presentation is a middle-aged, obese, prematurely gray dog with reduced activity, the only visible sign might be a patch of hair loss or a smelly coat.

While it seems that diagnosing hypothyroidism should be as simple as checking the blood level of the thyroid hormones, dogs don’t always read the same medical books veterinarians do, and they often throw us a curve ball. Usually, normal T4 levels mean normal thyroid activity, but low T4 levels do not always mean abnormally low thyroid activity. Sometimes, if the dog is ill when the blood is tested, the T4 levels will be low even though the thyroid is working normally. This is called Euthyroid (normal thyroid) Sick Syndrome. Additionally, several drugs will artificially depress T4 levels. The free T4, performed by a test method called Equilibrium Dialysis, is the most accurate means of testing for thyroid disease, and this method should be used if there is any question regarding the accuracy of the diagnosis.

SH levels are usually elevated in hypothyroidism, as the pituitary senses the low levels of T4 and T3 and keeps sending hormone to stimulate the gland. Once again, however, old Mother nature doesn’t always operate this way. Symptoms always trump blood tests, and sometimes the best thing to do is conduct a trial run with a thyroid supplement and see what happens. This is not a harmful procedure when dosed properly.

Treatment of hypothyroidism involves the once- or twice-daily administration of T4. Treatment is usually for life, and thyroid levels are monitored early in therapy to ensure proper dosage and make sure over-supplementation does not occur. If the thyroid levels are too high, symptoms such as restlessness, rapid heart rate, panting, and excessive weight loss may appear.

Periodic testing is a good idea, and the highest blood levels of thyroid will usually occur four to six hours after administration of the pill. Most often this is the proper time to draw blood. The lowest blood levels will occur just before the pill is due. If TSH levels are abnormally high during treatment, dosing may be inadequate.

Hypothyroidism is seldom a threat to your dog’s life, but correcting the condition should result in a healthier looking, more active, and more comfortable canine pal.

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