Hypothyroidism is the most common endocrine disorder in dogs. It occurs when the thyroid gland does not produce enough thyroid hormone. Since thyroid hormone affects multiple body systems, symptoms of hypothyroidism are varied and frequently non-specific. (HYPOthyroidism rarely affects cats, although cats are frequently HYPERthyroid; feline hyperthroidism will be discussed later this month.) 

The thyroid glands are located on either side of the trachea (windpipe). They produce thyroid hormones, which regulate metabolic function in cells throughout the body.

In most cases, hypothyroidism occurs because the thyroid gland is destroyed by the dog’s own immune system. It can also occur because of atrophy of the gland, a diet that lacks sufficient iodine, or rarely, as a birth defect.

Hypothyroidism can have many symptoms associated with many different body systems. Some of the most common symptoms are recurrent skin problems, frequent ear infections, cold intolerance, and weight gain or obesity that does not respond to typical weight management modalities.

Affected dogs may also exhibit lethargy or mental dullness. They may have hair loss, especially at the end of the tail, causing a classic “rat tail” appearance. The skin may thicken; this is often most noticeable on the face, where thickened facial folds cause a “tragic expression.”

Hypothyroidism can affect the heart, causing arrhythmias and a low heart rate. It can also inhibit electrical impulses in nerves throughout the body, resulting in weakness, nerve pain, a head tilt, and balance problems.

Hypothyroidism is often associated with increased blood cholesterol. High levels of fat in the bloodstream can lead to fatty deposits in the cornea, causing a condition known as corneal dystrophy. Although this is usually a cosmetic condition, it may indicate that testing for underlying hypothyroidism is warranted.

These symptoms are often suggestive of hypothyroidism, but they can also be associated with other disease processes, and are therefore not diagnostic. If your dog exhibits one or many signs of hypothyroidism, your veterinarian will run blood tests to confirm the diagnosis. The body’s feedback system that controls thyroid levels is complex, and involves many different hormones. An evaluation of several different hormone levels may be necessary to reach a diagnosis.

Treatment of hypothyroidism is fairly straightforward. It involves once- or twice- daily supplementation of thyroid hormone, which is given as a pill. Thyroid pills are usually affordable and fairly easy to administer.

Circulating thyroid hormone levels must be measured by your veterinarian at regular intervals until a therapeutic level is achieved. Then, levels are usually monitored once or twice a year. Treatment is life-long.

Most symptoms of hypothyroidism resolve within days to months of treatment. Owners of hypothyroid dogs frequently comment not only on resolution of clinical signs, but on how much happier and more youthful their dogs seem once their hypothyroidism is managed effectively.

Dr. Kelly Sulik owns and operates Animobile Mobile Veterinary Services in Tryon, N.C. She can be reached at animobiledvm@gmail.com.