Hypothyroidism in Dogs

Arnold Plotnick, D.V.M.

In mid-July, I was presented with a seven-year-old cocker spaniel for what I thought would be a routine annual examination. I began with questions about appetite, elimination, coughing and sneezing, and I was assured that Charlie was fine, although his owner reported that the dog had been drooling a little bit for the past two weeks. I made a mental note to do a meticulous oral exam. “The weird part is,” the owner continued, “he’s only drooling from the right side of his mouth.”

I began my physical. Charlie’s eyes looked fine, and so did his ears. I looked at the mouth next. Sure enough, saliva had accumulated in the corner of the mouth, overflowing the lips on the right side while the left side was fine. I continued the exam: heart and lungs normal, abdomen normal, skin no problems…everything checked out fine.

I returned to the mouth. Stepping back, I looked at the dog’s face as a whole. Not only was Charlie drooling a bit from the right side, but his lips on the right hung down farther than on the left. A light bulb started to go on in my head.

I touched the corner of Charlie’s left eye. He blinked. I touched the corner of the right eye. No blink. I gently poked at his left nostril. Charlie reflexively pulled his head away. I poked his right nostril, and he barely noticed.

Diagnosis: paralysis of the facial nerve on the right side. The cause? Unknown. For now.

Assured by the owner that Charlie had not suffered any trauma to the right side of his face, I recommended that we perform some routine blood tests and a complete thyroid panel. I explained that facial nerve paralysis has been reported in a small percentage of dogs with hypothyroidism. The owner agreed to the tests. Two days later, the results confirmed my suspicions. Charlie’s thyroid hormone levels were extremely low. Diagnosis: hypothyroidism, with secondary facial nerve paralysis.

Variable Signs and Symptoms

Hypothyroidism is the most common endocrinopathy (glandular disorder) in dogs. The thyroid gland, due to inflammation or atrophy of un-known origins, becomes incapable of producing an adequate amount of thyroid hormone, and dogs develop clinical signs of hypothyroidism. The signs are quite variable, because thyroid hormones are needed for normal metabolism, and deficiency can affect nearly all body systems, including the skin, the reproductive tract, the neuromuscular system and the cardiovascular system. Metabolic signs include lethargy, sensitivity to cold, mental dullness, unwillingness to exercise and a tendency to gain weight despite eating less food than normal. Because some of these signs progress fairly slowly, dog owners may unconsciously adapt to the changes in their pets and fail to recognize them as problems.

The most common abnormalities seen in hypothyroid dogs involve the skin and haircoat. A dry, dull, flaky coat is most often described, with hair that is easily pulled out. Hair loss often occurs in a characteristically symmetrical pattern on both sides of the body, sparing the head and legs; some dogs develop a “puppy coat,” as the guard hairs fall out preferentially, leaving the soft undercoat behind. Sometimes the hair loss only involves the tail, giving affected dogs a “rat tail.” Although skin and hair problems are more unsightly than health-threatening, signs that relate to vital organ systems, if undiagnosed and untreated, have the potential for serious illness.

Neurologic Signs

The nervous system has been well-documented to be affected by hypothyroidism. Lack of coordination, loss of equilibrium, paralysis of the larynx and dysfunction of the esophagus have all been described. Although rare, facial nerve paralysis has been described in approximately 4 percent of hypothyroid dogs. Charlie was odd in that this was the only sign that he had.

Cocker spaniels, golden and Labrador retrievers, dachshunds and Doberman pinschers are among the dog breeds that seem to have a predisposition to develop hypothyroidism. Middle-aged dogs are most often affected; the average age at onset is seven years.

Charlie is now on thyroid hormone replacement therapy. Medication is usually successful, with side effects uncommon. In eight weeks, I will perform another thyroid test to confirm that the medication has brought his thyroid hormone levels back within the normal range. I fully expect Charlie to be drool-free on his return visit, and to be blinking with both of his big brown eyes.

Dr. Plotnick is staff internist at the ASPCA Bergh Memorial Animal Hospital.

© 2001 ASPCA

ASPCA Animal Watch - Winter 2001