One of the tests that we ordered from Hemopet/Hemolife labs as part of their Thyroid 5 panel was TgAA, or thyroglobulin autoantibodies. Yeah, say that one three times fast.
Well, we ordered it, but I didn’t really understand what the measure was for, nor how to match it against the reference range; I just wanted to be thorough. To rehash, Bowdu’s test numbers from Hemopet looked like this:
- T4: 0.87 (reference range 0.80 ~ 3.80 μg/dL)
- Free T4: 0.53 (reference range 0.55 ~ 2.32 ng/dL)
- T3: < 10, verified by repeat analysis (reference range 30 ~ 70 ng/dL)
- Free T3: 1.9 (reference range 1.6 ~ 3.5 pg/mL)
- Thyroglobulin Antibodies: <1% (reference range: negative [?])
TGAA CONFIRMATORY TEST INTERPRETATION
less than 10% = NEGATIVE 10 ~ 25% EQUIVOCAL greater than 25% = POSITIVE
I didn’t understand why the reference range was supposed to be “negative,” since a number that’s less than 1% is still a “positive” number. At any rate, Bowdu’s TgAA results fell within the reference range, while the others were low enough to diagnose him with hypothyroidism. How did his TgAA relate to the diagnosis?
Well, I’ve done some reading and poking around, and so I’m going to take a stab at rendering my understanding of what TgAA is and what those test results meant. If any experts are reading this and found that I’ve misunderstood what I’ve researched, please let me know. This is NOT my field by a long shot!
Thyrogloblin AutoAntibodies (TgAA) are present in the system when the dog’s thyroid basically starts attacking itself, when something starts to go wrong with the thyroid glands. If a dog tests positive for TgAA, and other values are low, a dog is said to have autoimmune thyroiditis — the body is basically obstructing its own normal thyroid function. If other values are low, and TgAA is negative, it is labeled idiopathic thyroiditis — there’s some other reason that the thyroid production has atrophied. OR it could be that the condition has already progressed for some time such that you’re actually looking at the end stage of autoimmune thyroiditis, which is what I think is happening with Bowdu given his age and history of symptoms.
Some people called TgAA the “best” test in pinpointing thyroid dysfunction. As I understand it, it’s certainly not enough on its own. TgAA is the best advance indicator of possible thyroid disease further down the road, because if a dog is going to develop thyroid problems, he may test positive for TgAA as early as 1 year old, even without exhibiting other clinical symptoms or low T4 or T3 levels (subclinical thyroiditis). If TgAA comes back positive, it is “highly suggestive of later development of clinical hypothyroidism” (Ferguson, 658), though visible symptoms may not appear until years later. So it’s best to keep monitoring such a dog, and take into consideration all the other values from other thyroid tests.
In reality, the average pet owner is not going to ask for a TgAA test as part of annual bloodwork. But dogs intended to be used in breeding programs should have this measure on the record; it is one of the values that the Orthopedic Foundation for Animals (OFA) asks for when registering a dog in their database.
The diagnosis of “hypothyroidism” isn’t about a positive or a negative TgAA. But a positive or negative TgAA associated with hypothyroidism varies by breed. Hypothyroid English Setters and Golden Retrievers, for example, tend to test positive for TgAA more often than not, whereas hypothyroid Collies and Doberman Pinschers do not test positive for TgAA as often (Graham et. al, 624). Age at diagnosis was figured into those statistics somehow, which led researchers to suggest that the disease progresses differently, perhaps at a slower rate, in some breeds than in others. Overall, it’s about a 50/50 split across breeds whether you’re looking at autoimmune or idiopathic thyroiditis. You’ll have to consult the article or experts from your own breed club to see where your breed falls (there was no specific information for Shiba Inu, though Ferguson makes mentions the Basenji as one of the breeds with the highest prevalence of hypothyroidism based on some research done by Nachreiner at Michigan State University and others ).
This bit was interesting to me:
In a preliminary investigation of the influence of geography on the prevalence of thyroiditis in samples submitted to Michigan State University, some significant differences were observed. The prevalence of TgAA was significantly higher in samples submitted from North Dakota, Vermont, Wyoming, Minnesota, and Colorado compared with Michigan (range of odds ratios: 1.19 – 1.41; P < .05). The prevalence was significantly lower in samples from Massachusetts, Maryland, Virginia, North Carolina, Florida, South Carolina, Kentucky, Texas, West Virginia, Tennessee, and Alabama (range of odds ratios: 0.39 – 0.79; P < .05). There was no interaction with breed prevalence, but the underlying reasons (if any) for these observations have yet to be discovered. (Graham et. al, 623)
Tantalizing, isn’t it? Is it something in the water? Or the presence of puppy mills? Hunters? Urban sprawl? Mountain air? Who knows?!
At any rate, TgAA by itself is not an adequate test for diagnosing hypothyroidism. A dog can test positive for TgAA but still have T4 and fT4 levels within the normal range, which means the body is still producing enough hormone levels on its own, though the rumblings of worse to come are on the horizon. Likewise, a dog may not test positive for TgAA but the T4 and fT4 levels are low because the thyroids are already pooped; there’s nothing left worth attacking, so there’s no need to pump out more antibodies.
So I guess for the average pet owner like me, the TgAA was useful to suggest how far along Bowdu may have been in the disease. I’ll probably skip it next time, along with the T3 and fT3, and just order baseline readings of T4 and free T4 levels for the purposes of general monitoring.
Annotated list of References:
All articles come from The Thyroid special issue of Veterinary Clinics of North America: Small Animal Practice 37.4 (July 2007), edited by Cynthia R. Ward. I am disappointed to report that the veterinary college that delivered this title to my university library passed along a virtually pristine, unread copy. I sincerely hope that these articles are being accessed in electronic form by our future veterinary specialists in training!
- Graham, Peter A., Kent R. Refsal, Raymond F. Nachreiner. “Etiopathologic Findings of Canine Hypothyroidism.” Vet Clin Small Anim 37.4 (2007): 617-631.
[too technical for me in some parts, and doesn't really venture to offer any causes for the occurrence of thyroid disease though the authors tease their readers with intriguing statistics and suggestive avenues for further research; some illuminating charts which show some breed-specific information]
- Ferguson, Duncan C. “Testing for Hypothyroidism in Dogs.” Vet Clin Small Anim 37.4 (2007): 647-669.
[clear, easy-to-understand survey of current available testing methods, their usefulness, limits, and attendant problems in diagnosis]
- Scott-Moncrieff, J. Catherine. “Clinical Signs and Concurrent Diseases of Hypothyroidism in Dogs and Cats.” Vet Clin Small Anim 37.4 (2007): 709-722.
[also very technical, but covers a broad range and good for anyone who might be searching for answers on the relationship between thyroid disorders and other specific issues like cardiovascular and opthalmologic abnormalities, anemia, diabetes, bleeding disorders, etc.; good for mining additional sources]
To bring this all back home, here are some pictures of Bowdu’s recent dermatological issues. He’s lost chunks of fur on his neck, one armpit, and a little bit on his belly, though he hasn’t been itching excessively nor does he seem to be in discomfort though he’s bald to the skin in some places, and there are patches of hyperpigmentation which come and go. We’re still monitoring him and treating him with medicated shampoo on affected areas, as well as Sulfodene on the hot spots, which seems to be keeping things from getting worse.
If this is just “allergies,” as I’m afraid the vet is going to say, then allergy season is really starting earlier and earlier each year… I’m still unwilling to admit that allergy season is year round, though this may be our punishment for living in the land of perpetual sunshine and no snow.