First. let’s talk about the thyroid gland. Dogs and cats have a divided thyroid gland located on either side of the trachea just below the larynx. Humans usually have just one gland, more or less the shape of a butterfly. Some individual humans, dogs and cats can have ancillary thyroid tissue, usually small amounts, located along the trachea and airways. These are termed ectopic thyroid tissue and in some cases can maintain thyroid function if it is necessary to remove the thyroid gland surgically.
The thyroid gland is responsible for, or plays an important role in, many normal body functions. These include the regulation of body temperature, metabolism of fats and carbohydrates, weight control (both loss and gain), heart rate and cardiac output, normal function of the nervous system, growth and brain development in young animals, reproduction, muscle tone, and the condition of the skin and hair. So if the thyroid gland is not functioning normally, we can expect changes in these functions and those changes result in symptoms or signs of the disease.
Thyroid disease is manifest as either low or absent thyroid activity (hypothyroidism) or excess thyroid activity (hyperthyroidism).
Signs of hypothyroidism include; weight gain, lethargy, generalized weakness, mental dullness, alopecia (loss of hair that can be generalized or in spots), excessive shedding, poor new hair growth, dry and/or dull hair coat, excessive scaling of the skin, recurring skin infections and the inability to tolerate cold. In rare cases the animal may have seizures, a head tilt and infertility.
Signs of hyperthyroidism are, as one might expect, the opposite. There is a generalized increase in metabolism resulting in loss of weight despite an increased appetite. There is a general unkempt appearance and poor body condition. The animal may vomit and have diarrhea and frequently will be seen drinking water. This results in increased urine production. Some animals will have difficulty breathing and compensate with rapid shallow breathing. There is usually a rapid heart rate sometimes accompanied by so-called “gallop rhythm” — a type of abnormal beat. The animals are usually hyperactive, and often the thyroid gland is enlarged.
Hypothyroidism is most common in middle-aged medium to large breeds of dogs. The condition is rare in cats. It is more commonly found in middle-aged dogs four to 10 years of age. Anecdotal evidence seems to indicate that neutered males and females are at higher risk than intact animals. This condition is most commonly the result of inflammation of the thyroid gland or a decrease in active thyroid tissue from unknown cause(s). The condition can also occur as a result of treatment with the sulfa drug trimethoprim-sulfamethoxazole. In very rare cases, iodine deficiency in dogs can result in hypothyroidism but commercially prepared dog and cat foods all contain adequate levels of iodine. The treatment for this condition is replacement therapy with levothyroxine or another type of thyroid replacement.
The diagnosis of hypothyroidism usually requires laboratory testing that includes a complete blood count, biochemistry profile and urinalysis. Your veterinarian may be able to make an initial diagnosis based on the results of these tests, but it might be necessary to measure the levels of T3 and T4 and other endocrine lab tests. Your veterinarian may also recommend X-ray studies to check for other associated abnormalities.
Hyperthyroidism is the result of overproduction of thyroxin by the thyroid gland, usually the result of a thyroid gland tumor. It can also be an aftermath of inappropriate over-medication for hypothyroidism. It is rare in dogs but can occur. It is most commonly diagnosed in older cats, usually about 13 years old or older. Less than 5 percent of cats with hyperthyroidism are under 10 years of age. In addition to a thyroid tumor, hyperthyroidism can also be the result of congenital disease, iodine deficiency or the result of inappropriate therapy. Sometimes it is impossible to identify the cause.
The diagnosis of hyperthyroidism is often initiated by palpation of an enlarge thyroid gland during a physical exam and documentation of clinical signs suggesting this disease. This will usually lead your veterinarian to measure a thyroid profile that includes T3, T4, Free T4 and TSH in the blood. If the T4 is higher than normal the diagnosis is confirmed, however some early cases demonstrate T4 and the other hormone levels in the normal range. The performance of a T3 suppression test might be indicated and can produce a diagnosis. If the T3 suppression test results are still equivocal and if hyperthyroidism is still suspected, further tests including nuclear isotope imaging may be necessary to arrive at a diagnosis.
There are three types of treatment for hyperthyroidism: life-long oral anti-thyroid medications, surgical removal of affected thyroid glands and treatment with radioactive iodine. Tapazole (methimazole) is a specific anti-thyroid medication. This is a treatment that must be continued for the rest of the life of the animal unless surgical removal or radioactive iodine removal are indicated. Sometimes Tapazole treatment is used prior to surgery or radioactive iodine therapy to reduce thyroid hormone levels into the normal range to reduce the risk of surgery or radioactive isotope therapy. It is also indicated when the animal has congestive heart failure resulting from the hyperactive thyroid. Side effects from Tapazole include depression, vomiting, appetite loss and more seriously blood abnormalities. If surgical removal is the choice of therapy, the surgeon must be very careful to avoid damage to the parathyroid glands. Removal or injury to these glands will result in significant problems.
As always, if you suspect your animal has thyroid disease consult your veterinarian.
— By Dr. David Gross
Dr. David Gross graduated from Colorado State University’s veterinary school in 1960 and was in private practice for 10 years. He retired in 2006 as Professor and Head of Veterinary Biosciences, at the University of Illinois, Urbana-Champaign. Dr. Gross is the author of “Animals Don’t Blush,” which describes the unique patients and even more unique clients of a veterinary practice in Sidney, Montana in the early 1960s.