I’m Not Lazy...
If hypothyroidism is to blame, it’s best not to let sleeping dogs lie.
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We all think our dogs are the best, but let’s be real, sometimes they can be super lazy. While it’s normal for dogs to sleep for large portions of the day and you may find it relatable/inspiring, there is a point at which it becomes concerning. If you find yourself thinking what was initially cute is quickly becoming excessive, especially if weight gain is involved, this may not be run-of-the-mill sleepy pup syndrome. Instead, it could be a serious condition known as hypothyroidism.
What is Hypothyroidism in Dogs?
Fluctuations in hormones caused by inflammation or shrinkage of the thyroid gland cause hypothyroidism in dogs. Primary hypothyroidism is the most common hormonal disorder in dogs. The average onset of hypothyroidism in dogs is at seven years old. It is often found in large breed dogs — Doberman Pinschers, Golden Retrievers, Irish Setters, Great Danes and Boxers — and some smaller types, including Cocker Spaniels, Dachshunds, Poodles and Miniature Schnauzers.
Signs of hypothyroidism in dogs
The classic signs of hyperthyroidism in dogs include weight gain, lethargy, cold intolerance, skin and coat changes (yeasty-smelling fur; dark, lacy patches in the groin; excess shedding), vague and subtle symptoms that had snuck in slowly. Basically, a dog’s metabolism is working at a crawl.
Hypothyroid dogs tend to have elevated fasting cholesterol and triglycerides, elevated liver values, and mild anemia. Less often, they exhibit neurological weakness, poor appetite, corneal deposits, slow heart rate, heart arrhythmia, or infertility.
Causes of hypothyroidism in dogs
While overproduction of thyroid hormones is often closely associated with cancer, it is rare in dogs. Rather, more than 95 percent of hypothyroid canine cases are caused by unknown gland atrophy or immune destruction of the thyroid. When damaged, the gland cannot respond as well to the TSH (thyroid-stimulating hormone) secreted from the pituitary gland to make T4 (levothyroxine) and T3 (triidothyronine), currency used by the body for the metabolism of energy.
How Do You Diagnose and Treat Hypothyroidism?
Once diagnosed, hypothyroid dogs tend to respond well to a long-term, twice-daily dosage protocol of oral levothyroxine. Typically, within four to six weeks, they have more energy, they lose weight and their coats begin to shine. After T4 levels normalize, some dogs can be transitioned to a once-a-day medication schedule.
Sometimes, however, a dog’s thyroid disorder is atypical — their blood work doesn’t match the signs or vice versa. A dog may be suffering from other illnesses that lower T4, and certain medications can decrease thyroid levels as well. Adding to the confusion, for some dogs — such as Sight Hounds, Basenjis, or athletic dogs — thyroid levels are normally low. So how do we know if a dog is truly hypothyroid?
DCPAH thyroid test for dogs
To sort things out, veterinarians turn to what is currently the most thorough thyroid panel, offered by the Michigan State University Diagnostic Center for Population and Animal Health (DCPAH). Your vet may call this the “Michigan panel” though, even with it, the results are often less than crystal clear.
The Michigan panel evaluates TgAA (antithyroidglobulin antibodies), autoantibodies to the thyroid. A TgAA that is greater than 35 percent of the positive control value is positive for the presence of lymphocytic thyroiditis, but might not indicate full-blown hypothyroidism. According to DCPAH, “Positive values indicate thyroid gland pathology, and also tell us about the validity of the thyroid hormone results.” A caveat — TgAA can be elevated even when the dog is not yet truly hypothyroid. Because the gland must lose at least 60 percent of its function before clinical signs can be observed — which can take years, or may never happen — retesting a dog every six to 12 months is recommended.
To further define the results, DCPAH recommends testing fT4ED (free T4 by equilibrium dialysis) when “T4AA [a subset of TgAA] have been documented, non-thyroidal illness is known to be present, or the dog has received interfering substances such as steroids or phenobarbital.”
Some common meds can interfere with tests
Some medications lower T4 levels like phenobarbital, trimethoprim-sulfonamide, zonisamide, clomipramine and any glucocorticoid (drugs that stop inflammation, such as oral or topicals for ears, eyes and skin). So, if your dog is being given one of these medications, one way to clarify blood results is to work with your vet to safely wean him off the medication. After halting the medication for the prescribed period, blood work can be rechecked. If it’s not possible to stop a medication, a six-to-eight-week levothyroxine trial is sometimes employed.
In the end, after all the grey areas and questions, hypothyroidism comes down to this: treat the dog, not the blood work. Your dog’s vet can help you sort through the diagnostics and set up an appropriate monitoring schedule. Until then, don’t sleep on the warning signs.
Michigan Thyroid Panel
The Michigan Thyroid Panel measures many aspects of thyroid function. Here’s a tip sheet for decoding the results of your dog's hypothyroid test.
TT4: Total T4 measures both protein-bound and free T4 levels. Various tissues convert free T4 to T3 to be used by the body. T4 is commonly decreased in hypothyroid dogs, but concurrent illness and many drugs can also falsely lower it. However, if it is normal, it is very unlikely that the dog is hypothyroid, and no further testing is necessary.
fT4: Free T4 measures T4 not bound by protein (the form that’s converted to T3). It is often decreased in hypothyroid dogs. Concurrent illness and drugs can lower it.
fT4ED: Free T4 by equilibrium dialysis (rather than by direct radioimmunoassay, or RIA), separates the free T4 from protein-bound T4.The dialysate, which is the part of a mixture that passes through the membrane in dialysis (containing the free T4), is then assessed by RIA. The fT4ED is less affected by seizure medications, steroids and euthyroid illness, but it cannot be used to clarify ambiguities when a dog is on sulfa drugs. It is not affected by the presence of TgAA.
Total T3, reverse T3 and free T3: Free T3 is the active form of thyroid, converted from free T4 to be used by the body. Because they fluctuate independently of hypothyroid status, T3 results are less reliable indicators of hypothyroidism.
TSH: Thyroid-stimulating hormone is secreted by the pituitary to stimulate T4 when T4 is low from true thyroid disease, certain medications, systemic illness and levothyroxine withdrawal; 85 percent of hypothyroid dogs with low TT4 will have elevated TSH; those who do not make it harder to confirm the diagnosis.
TgAA: Antithyroidglobulin antibodies, which, when present at greater than 35 percent, suggest immune-mediated thyroiditis. These antibodies can affect TT4 and free T4 results, complicating diagnosis. Hypothyroidism may not be currently present, and it may or may not develop in the future.
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Sara Greenslit, DVM
Sara Greenslit, DVM, CVA, is a small-animal veterinarian and writer who lives and practices in Madison, Wisc.