Hypothyroidismis a common endocrinopathy (hormone-related condition) of dogs.
The word is believed to have been coined around the turn of the 20th century. Its Greek prefix, hypo- means “below,” the root is thyroid, referring to the glands in the neck that produce thyroid hormone, and ism means “a system of.”
Sometimes changes such as lethargy (low activity level), weight gain, weakness and thinning hair coat are mistakenly attributed to normal aging. Also, common signs of hypothyroidism in the dog are lethargy, alopecia (hair loss), recurrent ear infections and weight gain. Because thyroid hormone acts as the “accelerator pedal” of the body, when the hormone levels are low, everything slows down.
Lethargy is easy to understand with this analogy: If one fails to press down on the accelerator pedal of an automobile, it doesn’t go very fast. Hypothyroid dogs similarly lack the drive to “get up and go.”
Commonly-affected breeds include Golden retrievers, Doberman pinschers, dachshunds, Irish setters, Labrador retrievers, schnauzers, Great Danes, poodles, Cocker spaniels, Shetland sheepdogs and Boxers.
Adding to hypothyroidism’s tendency to cause lethargy, many hypothyroid dogs suffer from anemia, or low red blood cell (RBC) count due to inadequate stimulation of the bone marrow. Without enough RBCs to carry oxygen around the body, vigorous activity is impossible.
Other changes can also be lurking in the bloodstream. Lipid (fat) metabolism is affected, resulting in excessive cholesterol and triglycerides, which can lead to liver disease and pancreatitis, among other issues.
Weight gain can be insidious. Since these patients are not moving around as much, there can be a significant increase in weight without a concomitant increase in food intake. In fact, the hypothyroid dog may actually eat less and still gain weight.
Hair loss can result from a number of factors in these patients. First, with everything about the body slowed down, hair growth and replacement slow down, too. Hair follicles may enter the telogen phase prematurely, with the body sending the message that the hair is fully mature, when it actually isn’t. Such a defective hair shaft is subject to falling out prematurely, too, thus leaving the coat thin. The hypothyroid dog’s coat is often dry and dull.
Canine ear infections often are driven by the microenvironment of the ears. Excessive wax, moisture in the ears, hair clumping, foreign bodies, breed-specific factors and malformations of the ear canals can all contribute to recurring ear infections. However, if immune system function is “running too slowly,” as occurs in hypothyroidism, infectious organisms will not experience a vigorous response by the immune system, leading to problems that come back over and over. Furthermore, hypothyroidism itself changes the microenvironment, causing alterations to the formation of ear wax (cerumen) and other factors which may increase the likelihood of ear infections.
Diagnosis of hypothyroidism is made by a combination of physical examination and laboratory testing. A blood sample is first drawn for “Total T4.” This test measures the total amount of one form of thyroid hormone in the body. If this test result is equivocal, a second test for “Free T4 by equilibrium dialysis” (FT4ED) may be requested. FT4ED measures only the active form of the hormone as it is used by the body. Often we request both tests simultaneously, as Total T4 can be artificially lowered in a syndrome called “euthyroid sick syndrome.” FT4ED is not affected by euthyroid sick syndrome.
Once a diagnosis of hypothyroidism is made, twice-daily supplementation of thyroid hormone is in order. A “post-pill T4” test will be required a few weeks after beginning therapy to determine if the new thyroid level is too high, too low or just right. The blood sample must be obtained 4-6 hours after a pill is administered.
Thyroid hormone therapy is continued for life, with post-pill T4 tests being obtained every six months to allow the practitioner and owner to know if dosage changes are required.
Unlike people, high thyroid function rarely occurs in dogs, just as low thyroid function rarely occurs in cats. To read about hyperthyroidism in cats, click here.
See you next week, Dr. Randolph.