Dear Doctor – Sluggish thyroid, or faulty lab result?

Letter to Tufts Veterinarians


Q I took my four-year-old collie to the vet because he has been kind of sluggish since we took in a second dog last year. We thought he’d enjoy the company, but he seems depressed to have to share the spotlight, spending a lot of time under the couch. The vet wondered whether it’s hypothyroidism that’s making him sluggish and tested his thyroid with two different blood tests. One test for something called free T4 came back normal, but a test for another substance in the blood called TSH was a little high — 4.8, when the normal range is up to 4.2. The vet said the results show the dog has a sluggish thyroid — hypothyroidism — and that I should put him on thyroid medicine for the rest of his life. But since one of the two tests came back normal and the other was just a bit high, I’m hesitant. What do you think?
Joy Aten
Hudsonville, Michigan

Dear Ms. Aten,
A “Unfortunately,” says Tufts veterinarian Michael Stone, DVM, “some pretty good evidence has come out that all the tests we have for thyroid disease in the dog are not very good. A lot of the diagnosis is based on the ‘look, smell, and feel’ of the case as much as on the numbers. For instance, a vet will check to see if the dog looks a little ‘puffy,’ which could be a sign of the disease, and also look for other, more subtle clinical signs. So it’s hard to say without examining the dog whether he’s hypothyroid.”

That said, hypothyroidism generally does come on when a dog is young to middle-aged, and your dog’s age is consistent with that. Also, while the free T4 test that came back in the normal range for your dog is relatively sensitive – it measures the amount of thyroid hormone secreted by the thyroid gland in the neck – an abnormal TSH test can be the first sign that something is wrong. TSH stands for “thyroid stimulating hormone,” controlled by the brain. And if T4 secreted by the thyroid gland is low, the brain may call for more thyroid stimulating hormone to be released so that the gland revs up to put out more. Thus, the TSH result above normal. Over time, the thyroid gland will not be able to respond adequately to instructions from TSH, however, and T4 levels will fall below normal while the TSH levels remain too high.

At this point, says Dr. Stone, your dog “is in the gray zone” as far as a diagnosis, particularly because the introduction of a new dog is a confounding factor affecting his demeanor.

You have three choices. You can:

a) repeat the tests; b) wait and repeat the tests in, say, 6 months; or c) put your dog on thyroid medication and see the results. If a dog truly is hypothyroid, you should observe a dramatic change in his energy level within seven to 10 days. But sometimes, Dr. Stone cautions, “there’s a partial response, as if the dog has had caffeine. If the dog is not having a super-dramatic reaction, it’s probably just a speed-up effect — not enough to keep him on the drug.”

It’s tricky, Dr. Stone says. “We don’t like to over-diagnose and then have a dog on medicine for life that he doesn’t need. Complicating things even more is that there’s a lab that does a panel of blood tests with something like eight different numbers. All these tests are questionable in terms of their diagnostic value. Furthermore, when a vet runs eight different tests, you’re bound to find something in the so-called abnormal range that will justify a diagnosis of hypothyroidism.” He suggests passing on any suggestion for a large battery of tests to check solely for hypothyroidism.

The good news, Dr. Stone says, is that because hypothyroidism is not a fatal disease, it’s okay to wait to make a decision. You have time to re-test or to try the medication, and also for your veterinarian to make more clinical observations to help nail down a definitive diagnosis. A vet might also look at other things that suggest hypothyroidism – the presence of anemia and high blood cholesterol, for instance, along with values for “total T4” as opposed to just “free T4” — and enter them into the whole picture when making a determination.


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