Hypothyroidism

UPDATE: AITh risk & THAA measures

From the AKC-CHF report:

The clinical disease of AITh is an adult-onset condition, with first signs not appearing typically until 3-5 years. However auto-antibodies, a "prequella" to the clinical disease, may appear as early as a year, hence the value of testing for TgAA. Note that by the time the dog is a veteran, his autoantibodies have usually destroyed his thyroid, so testing for TgAA ceases to be useful, (no need for antibodies to something no longer present), but by this time, of course, the dog is normally on thyroid replacement and in any case, his values for such as T4 and TSH will be typically diagnostic otherwise.

The International Symposium on Canine Hypothryroidism recommends that TT4, TSH, freeT4, and TgAA (thyroid auto-antibody) are used in testing for AITh. Annual testing is recommended. There is currently no genetic test for AITh. There is, however, some preliminary data to suggest the inheritance may appear as a simple recessive, so perhaps a single MHC gene is at the heart of things. This is typically the case with autoimmune disease; a few very specific MHC genes are present in nearly all cases of autoimmune disease where such data has been collected. "The best current marker for this disease is a positive TgAA test result," reports Nachreiner, who goes on to say: "Assuming a single gene disorder and recessive trait, TgAA will be positive on in dogs having both genes for thyroiditis. TgAA will not be positive in the carriers (having only one gene for the trait), and many not be positive in some young dogs that become affected later." 

As to the genetics of the disease, the authors of the recent AVMA study state: "The finding that certain breeds were significantly more or less likely to have THAA in the present study is a _further_ indication of a familial predisposition for autoimmune thyroid disease and suggests that autoimmune thyroid disease has a genetic component." The majority (~90%) of thyroid disease in dogs is thought to be autoimmune related, so this means that genetics is at the bottom of this disease in dogs, and so breeder selection is key to reducing the incidence of thyroid disease in dogs. Also noted was that "females had significantly higher odds of being positive for THAA (TgAA) than did males," and "neutered males and females had a significantly higher prevalence of THAA (TgAA) than did sexually intact dogs." Here is an example of an "environmental" factor that affects the clinical picture of an inherited disease: intact animals are somehow protected (in part) from clinically significant hypothyroid disease, and females (as is typical with autoimmune disease) are, at the same time, a more susceptible to this form of disease sex (for some reason apparently not related to sex-hormones directly). 

Autoimmune diseases generally have their origin in the individual's inherited MHC--the immune genes that regulate self-recognition. In general the propensity TO autoimmunity, rather than the tendency to manifest a particular clinically-designated disease (like AITh), is what is inherited. This is like allergies: you inherit the tendency to be allergic, not allergies to specific substances. Essentially the immune genes inherited have a problem with discrimination between the "self" and the environment in
autoimmune disease. The inherited MHC cannot tell "you" from the "bad guys" and reacts aggressively to the body's own tissues/cells, producing antibodies which result in the death of tissues and even whole organs. You can have this sort of reaction generally, like with SLE ("lupus") or it can be confined to one single sort of cell/tissue/organ, like in thyroid (AITh) disease. This is a terrible dysfunction of the immune system, obviously, as it's raison d'être is to PROTECT you, so knowing "you" from "other" is an essential trait for it's proper function. 

The Gazette had a good article on the disease in the July issue of 2003. The AKC-CHF had a pamphlet on this topic offered in 2002. There is also a "white paper" from the International Symposium on Canine Hypothyroidism available for $5.00 through the AKC-CHF.