The thyroid gland performs a variety of functions, but is probably best known for its effect on regulating metabolism. Common in dogs and humans, hypothyroidism occurs when a dog's or person's body isn't producing enough thyroid hormone. In dogs this causes a wide variety of chronic symptoms, including lethargy, hair loss, a dull coat, skin problems, weight gain, obesity, anemia, high cholesterol and even a slowed heart rate or abnormal heart rhythms. As with humans, the symptoms are vague enough and non-specific enough that it's common for a dog to have the condition for several years before being diagnosed and treated.

Thyroid hormone is produced through a complex interconnected relationship between the pituitary gland, the hypothalamus and the thyroid gland. Problems resulting in hypothyroidism can occur anywhere within these three structures. More than 95 percent of all canine cases, however, are the direct result of damage to the thyroid gland, which in dogs is located in the throat. About half of all cases in which the thyroid gland has been damaged are due to an autoimmune response, meaning the dog's own immune system has for some reason turned on and attacked the cells of the thyroid gland. The second half of these cases is attributable to atrophy (decay) of the thyroid tissue, at which point it becomes infiltrated by fat. The cause of this latter problem isn't yet known.

In rare instances, hypothyroidism also can be a congenital problem, or else linked to iodine deficiency in the diet.

In dogs, hypothyroidism most often develops in middle-aged animals between 4 to 10 years of age. Mid- to large-sized dogs are commonly affected; the condition is rare in toy and miniature breeds. Those most likely to be impacted include the: Airedale Terrier, Boxer, Cocker Spaniel, Dachshund, Doberman Pinscher, Irish Setter and Miniature Schnauzer. For unknown reasons, German Shepherds and mixed-breed dogs are less likely to develop hypothyroidism. The disease also doesn't hit one gender harder than the other, although spayed females are more likely to develop the problem than those that are intact.

Hypothyroidism is diagnosed through a simple blood test which measures the level of multiple thyroid hormones. Even better, it is easily treated, usually through a daily dose of a synthetic thyroid hormone called thyroxine (sold under several brand names). How much medication your dog will need - and how often he will need it - depends on the severity of your dog's disease and how well he responds to the medication. Your veterinarian will draw blood samples periodically to check his response to the treatment, and then adjust the dose accordingly. The medication must be taken for life, but the good news is that your dog's symptoms should be completely eliminated.