Hypothyroidism is a syndrome caused by deficient thyroid hormone secretion. In most dogs, hypothyroidism results from the progressive loss of thyroid function. Two different mechanisms are most commonly involved, accounting for more than 95% of the cases of canine hypothyroidism. The first is an autoimmune disease in which the thyroid tissue is invaded by lymphocytes, and the second is an idiopathic atrophy, in which the thyroid gland is replaced by fat and connective tissue. Uncommonly, hypothyroidism may be caused by a thyroid tumor, or a congenital malformation of the thyroid.
Hypothyroidism is the most common endocrine disease in the dog, but the incidence is unknown. In general, middle-aged, larger-sized dogs seem to be more at risk for the development of hypothyroidism. Nine breeds have been shown to be at high risk, including Golden Retrievers, Doberman Pinschers, Dachshunds, Shetland Sheepdogs, Irish Setters, Pomeranians, Miniature Schnauzers, Cocker Spaniels, and Airedales. Though not proven, it is felt that hypothyroidism may be inherited. Intact males and females appear to be affected equally, but the risk of hypothyroidism is higher in the spayed female.
The two major thyroid hormones are thyroxine (T4) and triiodo-thyroxine (T3). T4 is produced by the thyroid, and T3 is derived from deiodination of T4. The production and release of T4 from the thyroid gland is governed by thyrotropin (TSH) which is produced by the pituitary gland. Secretion of TSH is governed by thyrotropin-releasing hormone (TRH) which is produced by the hypothalamus. Thus proper functioning of several body organs are necessary for adequate secretion and release of thyroid hormones.
Characteristic symptoms of hypothyroidism include lethargy, weight gain, hair loss, dry hair coat, intolerance to cold, and weakness. Some syndromes of neurologic dysfunction may also be related to hypothyroidism, including peripheral vestibular disease, laryngeal paralysis and megaesophagus. Hypothyroidism may also contribute to dilated cardiomyopathy and congestive heart failure in dogs. Hypothyroid male dogs typically have decreased testicular size and poor fertility or sterility. Females with hypothyroidism may show anestrus or poor fertility.
Unfortunately there is no single biochemical test that will conclusively confirm the presence of hypothyroidism. Most commonly, a serum sample is analyzed for the concentration of thyroid hormones, most commonly T4. A TSH stimulation test may also be performed in which the dog is injected with TSH, and a subsequent serum sample is analyzed for T4 and T3. However, there are breed differences in normal levels of thyroid hormones. It has been shown that Greyhounds and Scottish Deerhounds normally have considerably lower levels of T4 than do other breeds. Also, some drugs can cause the T4 level to be abnormally low. These include steroids, anticonvulsants, aspirin, phenylbutazone, and some antibiotics. Other biochemical indications of hypothyroidism includes elevated triglyceride and cholesterol concentrations and a mild anemia. Treatment usually consists of replacement hormone therapy.