Hypothyroidism and the Racing Greyhound By Dr Chris Petzel BVSc Technical Services Manager - Virbac Australia
The thyroid gland is one of the seven major glands that make up the endocrine system. Endocrine glands produce hormones that are transported by the blood system to all parts of the body. These hormones regulate many of the body's processes, such as reproduction, growth rate and metabolism. The thyroid gland consists of two lobes and is situated in the neck on either side of the windpipe. It is responsible for the production of the thyroid hormones, Thyroxine (known as T4) and Triiodothyronine (T3). Thyroid hormones regulate the body's metabolic rate which involves the digestive, circulatory and nervous systems. It therefore becomes obvious that a disorder of the thyroid gland can produce many different clinical signs. Hypothyroidism In dogs, hypothyroidism is a multisystemic disease. The clinical signs seen are a reflection of the effects on organs because of decreased metabolic function. Hypothyroidism arises when too little thyroxine is produced and circulated in the blood stream. This is caused by inflammation of the gland, autoimmune thyroiditis (which is probably an inherited condition), or the wasting away of the gland itself for reasons at present unknown. In the past it was thought that thyroid problems could be due to an iodine deficient diet (iodine is an essential component of the thyroid hormones). In developed countries, this is unlikely to occur as diets today (for humans as well as dogs) are generally made up of foodstuffs from a variety of sources and not just one area that may have iodine deficient soil. Enlargement of the gland, commonly known as goitre, can be caused by severe iodine deficiency.  If goitre  is seen in dogs,  it is usually caused by a tumour. Symptoms As stated previously, hypothyroidism can produce a large variety of symptoms but the classic signs are: Lethargy (poor race/training performance) Weight gain Hair loss Intolerance to cold Anoestrus or other fertility problems in the female Loss of libido and a decrease in testicular size in males The disease occurs most frequently in middle-aged dogs of mid- to large-sized breeds. Golden Retriever, Doberman Pinscher, English and Irish Setter, Miniature Schnauzer, Dachshund, Cocker Spaniel, and Airedale Terrier breeds have all been reported to be predisposed to hypothyroidism. Although Greyhounds are not on this list, there seems to be a high prevalence of the disease in this breed. Both sexes are equally affected and the disease is more common in spayed than in intact bitches. Normal racing Greyhounds have been reported to have lower T4 levels than other canine breeds, but higher T3. Often, the first an owner will notice is that the dog seems lazy, does not race as hard and puts on weight even though the food quantity has been reduced. In moderate to severe cases this weight gain can sometimes be seen on the head, making the skull look broader and the eyebrows heavier. Many owners just think that their dog is "getting old before its time" because often no other signs are noticeable. Skin and coat conditions are quite common - the most obvious being the loss of hair with excessive shedding, dryness and retarded hair regrowth is seen in 65% of dogs with the disease. Increased pigmentation of the skin can also occur. Balding patterns are usually bilaterally symmetrical with common sites being the lateral trunk, ventral neck and caudal thighs. Bald thigh syndrome may be a sign of thyroid insufficiency in Greyhounds as it is responsive to thyroid hormone supplementation. The nervous and cardiovascular systems are less frequently affected while loss of normal heat periods and infertility is common in intact females. Diagnosis The diagnosis of hypothyroidism can be difficult and the condition may often be over diagnosed. Your vet will take into account the clinical signs and then arrange for tests to measure the amount of the thyroid hormones present in the blood stream. Your vet should do a complete blood count and serum biochemistry (high blood cholesterol is an indicator). In Australia the most common diagnostic tool is to test the serum T4 concentration levels (a simple blood test). A low basal serum concentration of T4 is essential in the diagnosis of hypothyroidism. If the basal T4 is normal, the animal is not hypothyroid. However, the finding of low resting serum T4 is not absolutely diagnostic. Low values are also found in dogs with a variety of non-thyroidal diseases and in dogs treated with corticosteroids (dexamethasone), phenylbutazone, sulphonamides and anticonvulsants. Giving your veterinarian as much history as possible will greatly aid in the final determination. Treatment If hypothyroidism is diagnosed your dog should be rested and treated daily with thyroid hormone supplements. This drug is relatively inexpensive and generally well tolerated. Additionally it is recommended that you supplement your dog with vitamin A daily until clinical signs resolve, due to the involvement of thyroxine in the production of the vitamin. Feramo-GreyhoundÒ provides all the essential vitamins, minerals and trace elements required by racing Greyhounds, including vitamin A. Improvements in performance and the haircoat can often be seen within a few months of initiation of the hormone replacement. It is recommended that once or twice yearly blood tests be taken to evaluate the effectiveness of the treatment. Thyroid hormone replacement therapy will be lifelong in dogs diagnosed with hypothyroidism.