CANINE HYPOTHYROIDISM
The thyroid is an endocrine (hormone producing) gland, which lies adjacent to the larynx. Its function involves concentration of dietary iodine, and the synthesis, storage and secretion of the thyroid hormones triiodothyroxine (T3) and thyroxine (T4). The thyroid hormones are responsible for maintaining the body's basic metabolism including growth and development, thermoregulation, nervous function, etc. Thyroid hormones influence every organ in the body, and when they are out of whack, the metabolic defects can be extensive.
Hypothyroidism, or deficient levels of the thyroid hormones produced, is probably the most common endocrine disease of the dog. In most dogs hypothyroidism results from a progressive loss of functional thyroid tissue. This could be an autoimmune disease (lymphocytic thyroiditis) or by fatty and connective tissue replacing the thyroid gland tissue (idiopathic thyroid atrophy). Hypothyroidism occurs most commonly in dogs four to ten years of age. Breeds reported to be predisposed include the Airedale Terrier, Cocker Spaniel, Dachshund, Doberman Pinscher, Golden Retriever, Irish Setter and Miniature Schnauzer.
The overall function of thyroid hormones in the body is best exemplified by the metabolic defects associated with their deficiency. Dogs with inadequate levels of thyroid hormones are lethargic and tend to gain weight. They become intolerant to cold and may shiver excessively as they have difficulty maintaining normal body temperature. Dogs may experience changes in bowel function, such as constipation and diarrhea. Most obvious to the owners are the resulting skin and coat abnormalities. The coat becomes dry and hair production ceases causing bald spots (alopecia). The skin will be rough and dry, and myxedema (an accumulation of mucin) may develop, producing thickening of the skin, especially around the face and head. Reproductive problems, such as lack of libido and reduced sperm counts in males, and abnormal or absent estrous cycles with reduced conception rates in females, can also occur in dogs with hypothyroidism.
Accurate diagnoses of thyroid disorders are often difficult as the clinical signs may mimic other diseases (e.g. diabetes, Addison's disease). Laboratory tests should be performed and correlated with the clinical signs. The most common and sensitive measure is of total blood T4 by radioimmunoassay (RIA). While the vast majority of T3 and T4 in the blood are bound to plasma proteins, only the "free" hormones are physiologically active and responsible for maintaining euthyroid (normal thyroid) status of animals. If total T4 is low, free T4 should also be measured by equilibrium dialysis. Although less common, a test using TSH (thyrotropic or thyroid stimulating hormone) to stimulate release of T4 may also be useful. If the dog is hypothyroid, the TSH is elevated above normal while the T4 is low.
Normal Canine Thyroid Ranges
- T3 (Ng/dl) = 45 - 150
- T4 (µg/dl = 1.0 - 4.0
- TSH (Ng/ml) = 0.0 - 0.45
- Free T4 by ED (Ng/dl) = 1.0 - 4.0
Some researchers have implied that total T3 and T4 concentrations may be lower in the German Shepherd, Cocker Spaniel, Beagle, Boxer, Labrador Retriever, Alaskan Malamute and Siberian Husky. I've heard comments from a few veterinarians that the Norwegian Elkhound may also have lower concentrations than the "normal" range. Does this mean "hypothyroid" Elkhounds are misdiagnosed? Or, does it mean that Elkhounds have a greater chance to develop clinical signs of thyroid deficiency? Fortunately, administration of the synthetic thyroid hormone, sodium levothyroxine (L-thyroxine) will alleviate most of the above signs. Significant increases in alertness and activity is usually noted with the first 1-2 weeks of treatment. Complete hair regrowth may take 4-5 months.
Newer diagnostic tests for hypothyroidism include the search for antibodies (e.g. anti-thyroglobulin antibodies), which could indicate autoimmune thyroiditis, the genetic form of the disease. If these antibodies are found in a young dog, a follow-up is required to see if the dog develops hypothyroidism. Genetic factors have been cited to play a role in the origin of hypothyroidism in the Airedale, Boxer, Cocker, Dachshund, Doberman, Golden Retriever, Great Dane, Miniature Schnauzer, Irish Setter, Pomeranian, Poodle and Shetland Sheepdog. The AKC Canine Health Foundation is currently funding a study on thyroid disease, Active Grant No. 1450 Hypothyroidism - A Recombinant Protein Based Serological Assay to Measure Thyrotropin (TSH) for Assessment of Thyroid Function in the Dog. A specific antibody will be prepared from this immunogen to develop a diagnostic test.
Although solid epidemiological data to conclusively demonstrate breed and familial predisposition to primary hypothyroidism are lacking, many believe an association exists, and that it is most often probably an inherited disease. Given the potential for inheritance, it's difficult to justify breeding affected individuals, but because onset occurs later in life, carrier dogs may have already had litters of puppies prior to clinical diagnosis. It is our responsibility as breeders to test our dogs and bitches, keep good records, and join together to establish "normal" thyroid hormone ranges for Elkhounds.?