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About Hypothyroidism

What is hypothyroidism?
Hypothyroidism is a disorder of dogs in which the body does not produce enough thyroid hormone. In healthy dogs, thyroid hormone is produced by the thyroid gland, a small gland located in the neck. Thyroid hormone plays a very important role in the body in regulating the metabolism or normal functioning of the body. With insufficient quantities of thyroid hormone in the body, as in a hypothyroid dog, metabolism slows significantly.

What are the clinical signs of hypothyroidism?
As a consequence of the slower metabolism, the hypothyroid dog is lethargic or sluggish and therefore reluctant to go for walks. Not surprisingly, hypothyroid dogs tend to gain weight even though their diet has remained the same. Some dogs become more sensitive to the cold and will lie close to heating vents to stay warm. These signs however, may be missed or attributed to normal aging of the dog.

The most common reason that a hypothyroid dog is taken to a veterinarian for evaluation is because of changes in the skin and hair coat. Hypothyroid dogs tend to shed excessively but the lost hairs are not replaced. The skin becomes apparent beneath the sparse hair coat. Similarly, hypothyroid dogs that are clipped for surgery will not regrow their hair. In addition, the skin becomes dry and flaky. Some dogs will develop a “rat-tail” appearance due to the loss of hair on their tail. Usually the skin and coat changes associated with hypothyroidism are not itchy unless the skin becomes very dry. A darkening of the skin (hyperpigmentation) especially on the belly and the inner surface of the hind legs, can be a clue that that the dog is hypothyroid.

Which dogs are more likely to be affected with hypothyroidism?
Although virtually all breeds, including mixed breeds, can be affected with hypothyroidism, it is more commonly diagnosed in Labrador retrievers, golden retrievers, Doberman pinschers and cocker spaniels. Middle-aged dogs are more commonly affected between the ages of 2 and 6 years. This disorder affects male and female dogs in equal numbers.

How do dogs become hypothyroid?
Hypothyroidism in dogs is most commonly associated with destruction of the thyroid gland and a resultant deficiency in thyroid hormone in the blood stream. Two conditions in dogs can result in the destruction of the thyroid gland and are incriminated in over 95% of the cases of hypothyroidism in dogs. The first is called lymphocytic thyroiditis. In this condition, the body begins to produce antibodies against the cells in the thyroid gland which results in the destruction of the thyroid gland. It is not known why the body begins to produce antibodies against its own tissues, but there is likely a genetic component to this tendency. Signs of hypothyroidism may not develop until 75% of the thyroid gland is destroyed and this process is slow, taking from 1-3 years to progress.

The second condition that can result in destruction of the thyroid gland is called idiopathic atrophy. In this disorder, cells of the thyroid gland are replaced by fat cells. The cause of idiopathic atrophy is unknown at this time.
In less than 10 percent of cases, hypothyroidism is secondary, due to a lack of thyroid stimulating hormone (TSH). TSH is produced by the pituitary gland in the brain. Lack of TSH can occur as a congenital disorder (birth defect) where there is a malformation of the pituitary gland, or as an acquired disorder in adult dogs due to the growth of a pituitary brain tumor.