Hypothyroidism is the most common endocrine disorder in dogs and it seems that the Havanese is no exception. In a speech given by Dr. Jean Dodds, I was surprised to hear her make the statement that she is seeing large numbers of Havanese (and Havana Silk Dogs) with active cases of thyroid disease. If this is indeed the case, both breeders and Havanese pet owners need to understand what thyroid disease is, what causes it, and how to test for it.
One of the most difficult aspects of this condition is that it may not be apparent that a dog is afflicted with thyroid disease. A dog can have thyroid antibodies at an early age,but no apparent symptoms until up to seventy percent of the thyroid gland is destroyed. Significant to us as breeders is that these dogs may also have had many offspring, and like so many conditions, thyroid disease, especially that found in young dogs is thought to have a genetic component according to Dr. Niels Pedersen (genetics) of UCDavis.
First, let’s look at what the thyroid gland is, and what its purpose is. The thyroid takes iodine, found in many foods, and converts it into thyroid hormones. To do this Thyroblobulin (a precursor protein of the thyroid hormones) is produced by the thyroid epithelial cells and then combines with iodine and amino acids to make the active hormones called T3 (triiodothyronine; with 3 iodine molecules) and T4 (thyroxine; with 4 iodine molecules). T3 and T4 are then released into the blood stream, primarily in a form bound to thyroid binding proteins, whereas only a tiny fraction of these hormones circulates in the free, unbound form and is biologically active. These hormones travel throughout the body where they convert oxygen and calories to energy. Thus, every cell in the body is affected by the thyroid gland.
So what is hypothyroidism? It is simply a deficiency in the thyroid hormones usually caused, in dogs, by the development of antibodies against various components of the thyroid gland. This is known as autoimmune thyroiditis. These antibodies, TGAA (thyroglobulin autoantibody ), T3AA (triiodothyronine autoantibody), and T4AA (thyroxine autoantibody) gradually destroy the follicles in the gland which will eventually render the thyroid unable to function. The fact that it is a gradual process and that the disease often mimics other diseases, renders it more difficult to diagnose accurately. Without routine complete thyroid antibody testing (not just a T4 screening test) , the disease may go unrecognized for years or be misdiagnosed completely.
It is because the thyroid directly affects every organ in the body, that some symptoms can mask thyroid disease, and appear to be caused by what is, in fact another organ or tissue organ. Thus symptoms can vary widely, and include such things as lethargic behavior (lack of interest in play, frequent napping, tiring out on long walks), weight gain (sometimes without an apparent gain in appetite), skin infections, dry skin, greasy skin, skin odor, hair loss (especially on the trunk or tail) discoloration or thickening of the skin where hair loss has occurred, cold intolerance (seeking out warm places to lie down), slow heart rate, chronic ear infections, severe behavioral changes such as unprovoked aggression, head tilt, seizures, anxiety, compulsivity, and depression. Published data has also shown that megaesophagus, ruptured knee ligaments, testicular atrophy, cardiomyopathy, excessive bleeding, and corneal ulcers can be caused by thyroiditis and hypothyroidism. Classical symptoms of hypothyroidism typically appear only after 70% or more of the thyroid gland has been damaged by this progressive disease process.
As is so often the case with complex systems (not unlike the liver), thyroid diagnostics is not always easy, and does have some controversial elements to it. Dr. Dodds recommends testing for T4, freeT4, T3, freeT3, TgAA, T3 Autoantibody (T3AA) and T4 Autoantibody (T4AA) while OFA considers the results of just three tests to be sufficient in the diagnoses of thyroid disease, and deems the thyroid to be healthy if the free T4 (considered the gold standard), TSH and TgAA levels are normal. However, Dr. Dodds shows data to explain in several of her publications that this may underestimate cases of thyroiditis .
She stated in an e-mail to me, that in about eight percent of the cases of proven autoimmune thyroiditis, the TgAA could be normal while the T4AA and/or T3AA may be positive for a malfunctioning thyroid. Since most autoantibodies are directed against T3, according to Dr. Dodds, rather than T4, the 3-test OFA thyroid Profile would miss these cases completely. She also states that the OFA profile typically requires that the free T4 be measured by a dialysis step, which would remove the T4AA. Thus, the presence of those T4 antibodies would not be recognized.
The thyroid stimulating hormone called TSH which is released by the pituitary gland, is considered by Dr. Dodds to be a relatively poor predictor of thyroid disease in dogs (70%) versus humans where its accuracy is much higher (95%) . The relationship between the pituitary gland, and the thyroid gland in dogs is different to that found in humans. It would seem by reading the information on the OFA website that their methods use this reading to locate the lesion (thyroid or pituitary) rather than necessarily a predictor of disease.
So what’s the bottom line? Do we need to add another test to our arsenal? It’s not an inexpensive test but is it worth the cost? According to the latest publication by the Canine Health Foundation, “since proper thyroid function is critical to a healthy immune system, if the dog is susceptible to thyroid disease, by inference it is also likely susceptible to other autoimmune disorders. They are listed as being autoimmune hemolytic anemia, immune-mediated thrombocytopenia, hypoadrenocorticism (Addison disease), systemic lupius erythematosus, rheumatoid arthritis and myasthenia gravis. It also states in this same issue that dogs predisposed to autoimmune reactions are also more prone to reactions against parvo and distemper vaccinations. Maybe the main question should be, not whether to give the test or not, but which one, Dr. Dodds or OFA.
Thank you to Dr. Jean Dodd for her assistance writing this article.