Hypothyroidism is a multi-systemic disorder commonly seen in dogs and very rarely in the cat.  The disease occurs most frequently in medium to large size dogs that are middle aged.  Breeds which see greater incidence of the disease are: Golden retriever, Doberman pinscher, Irish setter, miniature schnauzer, dachshund, cocker spaniel and Airedale terrier.  Animals of both sexes are equally effected.

95% of hypothyroidism in dogs is caused by destruction of the thyroid gland itself.  5% of cases are caused by other abnormalities.  The exact cause of hypothyroidism is usually not of importance as treatment is generally the same regardless of cause. 

There are many clinical signs of hypothyroidism.  This is due to the body wide effects of thyroid hormone deficiency.  The most common clinical signs are: lethargy, alopecia/hair loss, weight gain/obesity, dry hair coat/excessive shedding, infertility of the female, cold intolerance, and other signs seen less commonly.  Dogs suffering hypothyroidism may exhibit one or several of the above clinical signs.

Diagnosis of hypothyroidism is based on the history and clinical signs of the patient as well as a blood test, which assesses the amount of thyroid hormone  (T4) present in the blood.  There are many other causes of decreased T4 levels so it is often necessary to run other blood tests.  Very low to non-existent T4 levels are diagnostic of hypothyroidism.  Slightly low levels of T4 are more difficult to evaluate and often trial treatment or further tests are necessary.

Treatment for hypothyroidism is relatively straightforward.  The dog is given a daily supplementation of T4 in tablet form.  The veterinary drug for this is called Soloxine.  The human preparation, called Synthroid, works well but is usually in a dosage form that is too small for dogs.  Synthroid is very expensive and generic preparations do not work well.  For these reasons, the veterinary drug, Soloxine, is the preferred drug for treatment of canine hypothyroidism.  Dogs with hypothyroidism begin with twice a day therapy.  After three weeks of therapy, a second blood sample is drawn four to six hours after receiving their morning Soloxine tablet.  The final dose of Soloxine will be based upon this "post-pill" test.  Treatment is most often life long.

Since many of the clinical signs associated with hypothyroidism take months or years to develop, reversal of these signs may take several months.  Results are not instant but they are rewarding.  In rare cases, no clinical improvement is seen.  In these cases, further diagnostics are necessary.