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The Mastiff and The Thyroid

By Dr. Robin M. Smith

My bitch has dark brown spots on her flanks, what is it? My dog hardly eats anything and she/or he is still overweight? My bitch doesn't seem to have normal cycles and I can't get her bred, why? My bitch was bred and confirmed pregnant by ultrasound but on her recheck at 30 days, the ultrasound showed evidence of resorption, why?

Many of you have had these same questions and are looking for answers. I believe there can be a multitude of causes for these problems and by all means your veterinarian is the first one for you to ask about your concerns. One of the causes for all of the above problems can be abnormal thyroid function. While I will talk about the thyroid and the diagnosis of thyroid problems and the treatment, I again prevail to you to seek your veterinarian's advice before doing anything. Sometimes, even if what I talk about is to you what you think is the exact thing happening to your dog, you could create more of a problem by not getting it accurately diagnosed.

Hypothyroidism is a syndrome characterized by deficient thyroid hormone secretion that can readily be treated with synthetic thyroxine (T4). Once the diagnosis is established, virtually all clinical signs and related disturbances can be completely reversed by T4 replacement therapy. In a small percentage of cases (<5%), however, reduced thyroid function occurs as a result of a more serious condition and recognition of the cause is at least as important as documentation of deficient thyroid hormone secretion.

Hyperthyroidism is rare in the dog and will not be considered here.

Hypothyroidism in most dogs results from progressive loss of functional thyroid tissue due to a primary problem with the gland. In the dog, there are two distinct mechanisms of thyroid destruction: lymphocytic thyroiditis, which is probably an autoimmune disease, and idiopathic (meaning "unknown") atrophy, in which the thyroid gland is replaced by fat and connective tissue. There are other less common causes which will not be discussed here, since the above account for about 95% of the cases.

Although not proven, genetic factors may play a role in the origin of hypothyroidism. In a major study in 15 U.S. and Canadian veterinary teaching hospitals, the mastiff was not among any of the dogs tested. In this test, strong evidence for genetic transmission of thyroid pathology in dogs was found in data from selected groups of laboratory Beagles, in which the cause was lymphocytic thyroiditis. These dogs showed a higher frequency of autoantibodies (antibodies produced against oneself) to some thyroid molecules. Therefore, although good data conclusively demonstrating breed predisposition to primary hypothyroidism is lacking, many believe that such an association exists and that at least with lymphocytic thyroiditis, this condition is probably an inheritable disease. Because the other primary cause, idiopathic atrophy, has not been linked to being heritable, it is hard to suggest sterilizing a dog unless the thyroid is biopsied and the diagnosis of lymphocytic thyroiditis is obtained. Also, the onset of canine hypothyroidism usually occurs later in life, after producing many puppies. With the advent of new diagnostic techniques, like the testing for autoantibodies, we may be able to determine without surgical intervention, whether or not one is dealing with lymphocytic thyroiditis or idiopathic atrophy. I will deal more with the diagnostic in a later paragraph.

The clinical signs of hypothyroidism can be subtle to being very overt. Signs include mental dullness (your dog may not be as dumb as you think), exercise intolerance, lethargy, poor haircoats, hair coat color change, hair not regrowing when shaved (especially noticed after a surgery), infertility, irregular estrous cycles, resorption of fetuses after being bred, neurological problems, bradycardia (slow heart rate), and cardiac arrhythmias (abnormal heartbeats). Not all of these symptoms will be seen, but whenever a breeder has a problem with reproduction, the thyroid should be examined.

Thyroid function and reproductive function have many interactions, many of which are not fully understood. In dogs, it has been shown that thyroxine (T4) is significantly higher during pregnancy than in any other reproductive state. We usually think of the females when we speak of reproductive problems, but males are affected also. Affected dogs have decreased testicular size and lower fertility than nonaffected dogs. Poor semen quality has also been reported. Infertility, prolonged anestrus, short estrus, and poor libido are reportedly associated with hypothyroidism in bitches. An increased occurrence of abortion, stillbirth, resorption and mummified fetuses have been reported also. But, it has also been found that reproductive dysfunction is NOT always found in hypothyroid bitches. In human women, hypothyroidism has been shown to cause irregular cycles, including ovulation failure or cessation of cycles. When conception did occur, spontaneous abortion, low birth weight and fetal death were common. It has been shown that pregnant women with clinical sings of impending spontaneous abortion who later did abort had lower T4 and T3 levels.

Where does this all leave us? Now that we know the thyroid can cause a lot of problems, what do I need to do? My recommendation as a mastiff breeder who is a veterinarian, is to have your dogs thyroid tested. The best place to send the thyroid tests at this time is Michigan State University. The reason I recommend testing all your dogs is that we do not have enough information on mastiffs and what is normal or abnormal. I have encountered bitches that have undergone resorption of fetuses, or low fertility rates, that have had normal thyroid function tests. I have also had dogs with the typical dark skin patches on the flanks have normal thyroid function tests. In all of these dogs, I have explored as many possibilities as I could to find other causes and have found none. After supplementing these dogs with thyroxine, the symptoms disappear and the bitches get bred and maintain their pregnancies. I am not saying that we should just arbitrarily put dogs on replacement therapy, but I am saying we need to look at what is "normal" for the mastiff breed. I believe if a particular breed or line of a breed has demonstrable signs of thyroid abnormalities, and all other causes have been eliminated, that maybe we need to look at an alteration of the "normal range" for thyroid function tests in that breed.

This is not the place to go into the physiological aspects of thyroid function. But I will say that there are more thyroid function tests than just the "T4" that many people test for. The actual thyroid hormone that is active in the body is T3. There is also reverse T3, free T4, bound T4, free T3 and bound t#, and circulating antibodies that can be measured and can help in diagnosing the problem. Michigan State tests all of these and give a good overall view of what is happening. A very important test, the antibodies produced, is important to know since these are often generated in association with lymphocytic thyroiditis, which we spoke of as possibly being hereditary. There is ongoing work to identify other important molecules, as TSH which once identified will lead to a new generation of thyroid diagnostic tests.

When diagnostic tests do not provide a clear diagnosis, thyroid replacement therapy has been suggested as a valid diagnostic step in an animal suspected of being hypothyroid. Again, every attempt should be made to rule out nonthyroidal illnesses using history, physical examination, routine laboratory, and other appropriate testing before doing this. Your veterinarian is the best judge for this trial.

I believe that we have a lot to learn about the mastiff and the thyroid problems encountered in the breed. I am trying to collect information on as many mastiffs as I can and their thyroid profiles. Again, one must know what the "normal" is before we can diagnose the abnormal. I would appreciate your input and any thyroid test information that you have on your dogs as I am trying to put together information. The more I have, the more valid the information and the more we can all learn from it. If anyone has any questions regarding thyroid problems or just wants information, please feel free to contact me. Again, I am learning also and some of you have had much more experience with the breed and their particular problems.

Next issue, I hope to address breeding recommendations, as well as how best to know when to breed your bitch and other reasons your bitch may not become pregnant when you wanted her to be.

Please send inquiries to: Dr. Robin M. Smith: 490 E. Watersville Rd., Mt.Airy, MD 21771

Some of the information in this article has been taken from: "The Veterinary Clinics of North America. Small Animal Practice." May,1994

(Dr. Smith has spent the last 10 years in veterinary practice, the last year specifically in emergency medicine. Prior to that she was in the nursing field. Her association with mastiffs as a breeder started about six years ago and she has also been spending time specializing in mastiffs from a veterinary perspective. We appreciate her allowing us to reprint her articles. References made to "future" articles apply to another publication, but we will have more from Dr. Smith in the Reporter at a later date).

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