Canine Hypothyroidism

Hypothyroidism is the clinical condition caused by thyroid hormone deficiency. The thyroid hormones, thyroxine (T4) and triiodothyronine (T3), are produced by the thyroid glands located in the neck on either side of the trachea. Thyroid hormones influence the metabolism of most of the organs in the body. Deficiency of thyroid hormone results in a decreased metabolic rate, which may cause a wide variety of clinical signs. "Decrease in metabolic rate" means that the speed at which cells function or "work" slows down. For example, the heart rate slows, mental function slows, and body temperature decreases. Decreased secretion of thyroid hormones by the thyroid gland may be due to inflammation (thyroiditis) or progressive failure (atrophy) of the thyroid glands. In rare cases, thyroid tumors may cause hypothyroidism. Hypothyroidism is the most common endocrine disease of the dog. Breeds that are predisposed to developing hypothyroidism include the golden retriever and the Doberman pinscher, but any breed of dog may be affected. The most common clinical signs of hypothyroidism are weight gain, cold intolerance, lethargy, and a variety of skin problems. The most common skin abnormalities include hair loss, changes in hair color and quality, and predisposition to skin infections. Other less common clinical signs include abnormalities of the reproductive and nervous systems. A diagnosis of hypothyroidism is mad by measurement of thyroid hormone concentrations (T3, T4, and free T4) in the blood. If thyroid hormone concentrations are low, other tests may be performed to determine whether the decrease is due to a thyroid gland problem or to the effects of other diseases or medications. These additional tests may include measurement of thyroid-stimulating hormone (TSH) and measurement of a variety of antithyroid antibodies. In some cases, it is necessary to use trial therapy with thyroid hormone supplementation to confirm the diagnosis of hypothyroidism. Fortunately, hypothyroidism is a disease that is easily treated. Treatment involves daily or twice-daily oral medication with synthetic thyroxine. Treatment is usually started with two treatments per day, one in the morning and one in the evening. When the clinical signs have resolved, the treatment can be reduced to one dose per day in many dogs. In most cases, treatment is required for the life of the dog. It may take several weeks for the clinical signs of hypothyroidism to resolve. An increase in activity level is usually observed after 1 to 2 weeks of treatment. It usually takes longer for skin problems to resolve, but improvement should be observed within 6 to 8 weeks. In some cases the skin may actually appear worse for the first 1 to 2 weeks of treatment as the old haircoat is shed. If a dog was experiencing reproductive or neurologic problems, it may take several months of treatment for complete resolution of the problems. Clinical signs of excessive supplementation with thyroid hormone include nervousness, weight loss, and increased drinking or increased urination. These signs are rare, but if they occur it is important to call your veterinarian for adjustment of the dose. In order to establish that the dose of thyroid hormone supplementation is appropriate, it is recommended that blood samples be collected for measurement of thyroid hormone concentrations 1 to 2 months after the start of treatment. The results of these tests are used to adjust the dose of thyroid hormone supplementation. Then it is usually necessary to measure thyroid hormone concentrations only once a year or if clinical signs of hypothyroidism recur. The above is general veterinary information. Do not begin any course of treatment without consulting your regular veterinarian. All animals should be examined at least once every 12 months.