Talking About Thyroid
Everything you didn’t know you needed to know
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 lower 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, in 2004 the Ridgeback ranked #4, with 21.1 percent of evaluated dogs testing positive, in Michigan State University results compiled by the OFA. (The Ridgeback clocked 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 funded a study of thyroid-positive dogs, where Dodds coordinated the shipment of samples on U.S. RRs to the U.K. for inclusion in this study, in the hopes of finding a genetic marker. (A second phase of this study is underway. Click here to find out how you can contribute.)
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 can signal hypothyroidism.
Free T4 by dialysis. 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 and steroids, it is a fairly reliable number.
As the autoantibodies 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 can 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, hormones and other factors.
TSH. When there is an inadequate amount of FT4D in the bloodstream, the pituitary gland senses this and begins secreting TSH, or thyroid stimulating hormone, 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, the output for 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 a 70 percent predictability rate. “You can get falsely high readings in normal individuals, and falsely low in hypothyroid individuals,” Dodds says.
Of the three OFA values, Dodds considers TSH by far 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. The average dog becomes hypothyroid between the ages of 2 and 6. But in some lines, Dodds notes, dogs can be veterans age 8 or 10 or even 12 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 2 and 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 Canine Health Information Center, or CHIC, suggests thyroid testing at ages 2, 3, 4, 5, 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 “normal” for that 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 thyroid’s going to be wiped out” and along with it his FT4D reserve in his bloodstream, 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 and T4 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 autoantibody results (TgAA) come back normal when they are in fact elevated. The test simply fails to pick them up, Dodds says, though they are there.
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 autoantibodies and your dog has 201?” Dodds asks. “You need to know that number.”
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 who never “turn” hypothyroid that is, while the antibodies are circulating, the FT4D and TSH levels stay normal.
Dodds’ position is that 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 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 dog 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 normal, “which tells me right away that their bodies are using 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.
This article originally appeared in The Ridgeback Register. © Denise Flaim
If breeders are unsure about how to read a thyroid panel, or need more expertise than their regular vet can provide, Dodds can test blood and interpret the results at HemoPet, her Santa Monica, Calif., laboratory. (For those who want OFA clearances, Dodds can then forward a sample to the lab of your choice.)
For more information, including testing instructions and blood-test request submission forms, visit www.itsfortheanimals.com/HEMOPET.HTM.
Testing Intact Bitches
Because hormones can affect all kinds of health screenings, from hip X-rays to thyroid panel, intact bitches should only be tested during anestrus, which is the “quiet time” between heat cycles.
According to Dodds, breeders have a month-long “window” for this, starting 12 weeks after the first day of the previous heat cycle. So exactly three months to the day after your bitch first comes into season, and for 30 days thereafter, is the only time you should schedule health screenings for her.
The reason? While it may be many months before she comes into season again, after that brief anestrus period, your bitch’s body will begin to prepare her for her next heat. While these changes are “silent,” they are real, and might prevent you from getting an accurate assessment of factors such as joint laxity or thyroid values.
Interpreting Equivocal Thyroid Results
Dogs whose laboratory values do not fall within the range that the OFA considers normal but whose values are not clear-cut enough to label them hypothyroid will get results back that say “equivocal,” which is a fancy way of saying, “We don’t quite know yet.” Here is how some of those “iffy” results may be interpreted.
FT4D -- LOW
This dog could just simply have a low FT4D because that’s just normal for him, or because his active lifestyle as a performance dog is constantly depleting his levels of thyroid hormone. However, another explanation is that the thyroid test “missed” the rise in autoantibodies levels (TgAA), which, now that the thyroid is destroyed, have gone back down again. So is he or isn’t he? In the absence of earlier “baseline” thyroid panels, more testing is needed.
FT4D -- NORMAL
In hypothyroid dogs, the TSH is too high, not too low. While there is the relatively unlikely possibility that his dog has some disorder of the pituitary gland, that “off” value probably is not an indicator of hypothyroidism if all the other values are normal.
FT4D -- NORMAL
TgAA -- HIGH
This dog is not technically hypothyroid. And certain factors, such as recent vaccination, can raise TgAA levels, so retesting is critical. But if TgAA levels remain high, the FT4D level likely will drop and the TSH will spike, and the dog will become hypothyroid.
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