By Denise Flaim
Consider these two fictional Ridgeback breeders:
Both own intact males that passed their OFA thyroid at 1 year old. Now, three years later, both want to use their dogs at stud for the first time.
Breeder A, knowing that his male has an OFA thyroid clearance, simply breeds his dog.
Breeder B decides to re-test. But this time, one of the values on the thyroid panel is off: The dog’s TSH level is higher than the range that OFA requires to give the dog a clearance. Knowing that the panel’s other results are normal, Breeder B breeds his dog anyway.
Given these two scenarios, which breeder could unknowingly be contributing to the burgeoning thyroid problem in our breed?
If you say Breeder B, keep reading.
As awareness of inheritable thyroid disease, or autoimmune thyroiditis, has increased in our breed, so has the confusion over which dogs are acceptable breeding prospects, and which ones are not. One thing everyone can agree on, however, is that the problem is not going away.
“Ridgebacks are clearly in the top group” of hypothyroid dogs, “and that has happened relatively recently, in the last five years,” says Dr. Jean Dodds, DVM, arguably this country’s foremost authority on thyroid and autoimmune disorders.
While no breed can surpass the English setter, which has more affected dogs than any other, the Ridgeback ranks #4, with 21.1 percent of evaluated dogs testing positive, in Michigan State University results compiled by the OFA.
(The Ridgeback clocks in at a lower #6 with 10.4 percent positive in the OFA’s overall statistics, which include all the labs from which it accepts results. But Dodds considers the MSU ranking to be more precise, because MSU lab processes the largest number of thyroid panels.)
On a breed-club level, the Ridgeback community has been proactive in its approach to this health challenge. The Rhodesian Ridgeback Club of the United States has funded a study of thyroid-positive dogs, which Dodds will coordinate, in the hopes of finding a genetic marker.
But until a DNA test is developed to identify carriers, it’s up to individual breeders to make educated, responsible decisions in limiting, if not stopping the spread of this autoimmune disease in our gene pool.
Deciphering the Thyroid Panel
The first step for breeders is to understand what the values on a thyroid panel mean. And oftentimes, it is not enough to rely on your local vet to help you make interpretations of it for your breeding program.
“The average clinical veterinarian doesn’t really understand what the tests are measuring,” Dodds says about the OFA thyroid panel. “He or she may not know what the predictive nature of those three values are.”
The trio of results on which the OFA Thyroid clearance are based are:
TgAA. Hypothyroidism is, simply put, an underactive thyroid gland. In autoimmune-mediated hypothyroidism, also called autoimmune thyroiditis, which is the heritable kind we are discussing here, the dog begins to develop a high number of Thyroglobulin Autoantibodies (TgAA), which seek out and kill the thyroid gland.
This antibody level is measured in the TgAA value on the OFA panel. High TgAAs in at-risk breeds usually signal impending hypothyroidism.
Free T4 by dialysis (FT4D). This value measures the amount of thyroid hormone that is circulating in the bloodstream and available, or free, for the body to use. Of the three values the OFA measures, the FT4D is the “most accurate biologically,” Dodds says. While the value can be affected by certain drugs, such as anticonvulsants, it is a generally reliable number.
As the thyroid autoantibodies and associated T-lymphocytes destroy the thyroid, the level of FT4D begins to drop, because the damaged thyroid cannot produce an adequate amount of hormone. So low FT4D values often signal hypothyroidism.
Do not confuse free T4 by dialysis with total T4. Total T4, which represents all the thyroid hormone in the dog’s body, can fluctuate significantly, depending on the dog’s stress level, other hormones, illness, certain drugs, and other factors.
Thyroid Stimulating Hormone (TSH). When there is an inadequate amount of FT4D in the blood stream, the pituitary gland senses this and begins secreting TSH, which does precisely what its name implies: It stimulates the thyroid to produce more hormones.
“As the pituitary gland keeps trying to stimulate the thyroid gland, TSH output from the pituitary gland goes up,” Dodds says. “Ideally, an elevated TSH should be a very important predicator of primary thyroid disease.”
The problem is, the TSH value only has an average 70 percent predictability rate in dogs, in contrast to humans. “You can get falsely high readings in normal individuals, and falsely low ones in hypothyroid individuals,” Dodds says.
Of the three OFA values, Dodds considers TSH to be the least reliable.
Once Is Not Enough
The values in a thyroid panel are inter-related, and they can change significantly as the dog matures, or develops thyroid dysfunction. The average dog becomes hypothyroid between the ages of 1 ½ and 6. But in some lines, Dodds notes, dogs can be very young (8 to 14 months of age) or veterans (8 or 10 or even 12 years old) before they develop autoimmune hypothyroidism.
(For those who might think that having late-onset hypothyroidism in veteran dogs might not be such a bad thing, Dodds notes that she has studied several lines of dogs that bred such late-affected veterans. In subsequent get, the hypothyroidism manifested earlier and earlier. By the fourth or fifth generation, dogs were becoming hypothyroid at age 1 ½ to 3 – a trend she notices among hypothyroidism-prone breeds in general.)
The solution is clear: Breeders need to recognize that a normal OFA Thyroid is not a permanent pass. Like a CERF clearance, the thyroid panel needs to be repeated regularly to confirm the dog or bitch’s breedability. (The OFA recommends, but does not require, that re-examination occur at ages 2, 3, 4, 6 and 8 years.)
“People get a CHIC number and they think they don’t have to be doing testing again. It’s human nature – ‘I had a blood test, my dog was clear,’ Dodds says, referring to the Canine Health Information Center, a centralized canine health database. “That’s part of the problem.”
Ideally, Dodds says, breeders should test their dogs annually until age 9 or 10, and thereafter only if the dog displays symptoms. This systematic testing not only will “catch” a dog before he or she becomes hypothyroid, but will also give the breeder a “database” that will show what is truly “normal” for that individual dog.
And older dogs who stay autoantibody-negative their entire lives will become increasingly important to breeding programs. “We need their semen frozen,” Dodds concludes.
What Is ‘Normal’?
The OFA Thyroid ranges are cut and dried: If any of your dog’s values falls on either side of the stipulated ranges, he or she flunks. Period, end of story.
But Dodds argues that each dog should be looked at on a case-by-case basis, allowing for the fact that they are individuals.
“Remember that the OFA ranges are based on the average pet, who’s a couch potato and doesn’t do much more than walk around the yard,” she says. “When you’ve got breeding stock that’s going to perform, they’re quasi-athletes and they’ve got to be different” in terms of the lab values they produce.
For example, in the case of a Ridgeback who lure-courses every weekend, “his blood thyroid values are going to be quite low” and along with it his FT4D reserve in his blood stream, Dodds says. And some breeds, especially sighthounds, have low FT4D values to begin with.
Another criticism Dodds has of the OFA Thyroid clearance is that is looks only at those three values. Other values from a “full” thyroid panel – including total T4, T3, freeT3, T4 autoantibodies, and T3 autoantibodies – are ignored.
“As one is doing research and looking for a gene, you want to get more tests, not fewer,” says Dodds. “When you hunt and peck for certain values, you very well might find that five or 10 years down the road, you should have been testing the other parameters as well.”
Taking a comprehensive look at a dog’s full panel is even more important when you consider that 4 percent to 8 percent of thyroglobulin autoantibody results (TgAA) come back normal when the animal has either T3 and/or T4 autoantibodies and thyroiditis (false negative TgAA results). The test reagent simply fails to dentify these dogs, Dodds says, although they have thyroiditis.
On a related note, be sure to use a lab that will give you the actual values of your dog’s test results. For example, Cornell does not provide a number for the TgAA, noting instead only if the result was “high” or “normal.” “What if normal is up to 200 percent autoantibodies and your dog has 201 or 199?” Dodds asks. “Are these really different? Of course not. So you need to know that number.”
‘Vanishing’ Hypothyroidism
Some dogs may exhibit a temporary spike in TgAA levels after receiving a rabies vaccination, after which the levels go back down. But Dodds notes that some dogs with sustained high autoantibodies never “turn” hypothyroid – that is, while the antibodies are circulating, the FT4D and TSH levels stay normal, and they remain clinically healthy.
Dodds’ position is that in affected families or high-risk breeds, if these autoantibody-positive dogs are not hypothyroid, they eventually will be. “Most people are waiting for the fat, hairless, smelly dog, which is a year or more down the road,” she says. Instead of waiting for the autoantibodies to destroy the thyroid and then affect the FT4D and TSH levels, Dodds advocates removing the dog from the breeding program and starting him or her on thyroid medication.
Dodds notes that other veterinarians might disagree with this approach, arguing that the dog is not yet “officially hypothyroid.” But she notes that too many behavioral problems she has seen – especially unprovoked aggression that might lead to a dog being euthanized – are often linked to this very early stage of thyroid disease. Also, when dogs that are autoantibody-positive but otherwise normal are treated with thyroid medication, they do not become hyperthyroid – that is, their thyroids don’t become overactive. Instead, their values stay within the expected therapeutic ranges, “which tells me right away that their bodies are utilizing the hormone you’re treating them with.”
In the end, says Dodds, breeders need to raise their own personal bar and begin strenuously screening for a disease whose complex nature often makes it difficult to detect and predict. It could take up to two years for researchers to isolate the genes that cause hypothyroidism in the Rhodesian Ridgeback. And, considering the grim statistics, it is time we cannot afford to waste.
(c) Denise Flaim 2004. May not be copied or reprinted without the author's express written permission.
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