HYPOTHYROIDISM

Dogs with hypothyroidism have impaired production and secretion of thyroid hormones resulting in a decreased metabolic rate.  The disorder may be acquired (a progressive deficiency of thyroid hormone) or congenital (present at birth).  The acquired form is the most common in dogs and appears to be widespread in Alaskan malamutes, though we need more data to determine its exact prevalence.

Found most commonly in dogs aged four years or older, the disorder is the result of gradual atrophy of the thyroid gland or progressive replacement of the thyroid gland with lymphocytes due to an autoimmune process (lymphocytic thyroiditis).  The disease tends to run in families and is therefore thought to be genetic, though the exact mode of inheritance is unknown.  Affected dogs should not be bred.

A broad range of clinical signs make hypothyroidism a challenge to diagnose. Early signs include lower energy levels, unusual episodes of aggression, and increased susceptibility to infections.  As the disease progresses, the dog may develop symmetrical hair loss, darkening of the skin, or dry or greasy hair.  Other clinical signs include a slow heart rate, lethargy, difficulty maintaining body temperature, mental dullness, exercise intolerance, infertility, constipation and weight gain.  A dog may exhibit all or only a few of these symptoms.  When hypothyroidism is suspected, ask the veterinarian to do a complete thyroid assay.

Standard treatment consists of thyroxin supplementation once or twice a day for life.  Within a week of starting treatment, the dog's attitude and activity levels should improve, although improvement in the skin and coat can take up to six weeks or more.  With treatment, all symptoms should eventually disappear.  If they do not, consider whether your dog may have been misdiagnosed.  Because the symptoms are similar to those present in a variety of other disorders, hypothyroidism is among the most overdiagnosed of canine diseases.

Developed from information contained on the 1998 Canine Inherited Disorders Database Web site and from Merck Veterinary Manual.